As literally the first character of interest you see upon loading the game, and a legend in his own right, William is naturally going to be a character a lot of people choose as their first attempt at the game. However, this is Paradox Interactive; bear in mind that interesting does not correlate with easy in the developers minds.
William the Bastard is a deceptively difficult character to play. On the surface of things, he looks like he has an easy job, and it's certainly true that of the three claimants to the throne of England in 1066, William has the easiest task.. Harold has the smaller army, and he has to face down Harald Hardrada before he can get to grips with William. However, the fact remains that William is the attacker, and he is facing a very tenacious, determined enemy. On top of that, even if William is victorious, and he will be given time, he must still contend with a very angry Hardrada, to say nothing of the lords of England whose elected leader William has forced aside. Many players will rush into the scenario only to find that this is not the easy break in to warfare that they had imagined.
Before you begin any war, it's important to consider exactly what your strengths and weaknesses are. William has two massive advantages over Harold Godwinson and Harald Hardrada. The most significant of these is that he has a huge army; William typically starts with 11,000 men if you are the player, with Godwinson only starting with 10,000 and Harald a paltry 8,500. This means that you should win any direct conflict as long as you don't do anything foolish.
Secondly, you have about a thousand gold starting off; by the standards of the game, this is a vast fortune to possess, especially for a duke. And as you're on the offensive, you'll be taking castles and cities and looting them all the way through; as long as you keep going, the war will more or less pay for itself (I wound up making a profit.)
However, there are numerous disadvantages William faces. For starters, he is the attacker, and this is the most significant factor in your decision making process. William has 11,000 men, but victory over his two opponents also means actually physically taking over large chunks of the country and forcing Harold to concede control to you.
It also means you're going to have to take a long time over this. It took me until 1071 to get Harold to capitulate; admittedly this is because I did it through slow sieges to preserve my army rather than spamming assaults. Don't go into this expecting to replicate the easy capitulation of England achieved in history; your chances of both William killing Harold and Harold killing Harald are microscopic.
A major consideration is that you are not fighting for Normandy, but for William. This means that you MUST keep him safe; if the major battles are over consider getting his part of the army sent home. In both my campaigns (the first of which was unsuccessful... and this was after winning as Godwinson!) William wound up being made uncapable during the battle due to a particularly aggravating event where your liege is "struck in battle and won't stir from his repose." Brilliant, the Conqueror is in a coma. And, if the time it took him to die is any length of time, he won't be around to complete his conquest much longer. Don't take unnecessary risks with your characters life; Normandy will never get this opportunity again, and the war will not continue with Williams death (he'll pass down his claim though, but your chances of conquering a Kingdom with a Duchy, well...)
Consider what alliances your enemies make. I had to fight Svend of Denmark (who can also come into the war of his own accord; he has a claim as well!) when he came in as Hardradas ally; he more or less added another 3000 or so soldiers to Hardradas army, although thankfully Hardradas force had been defeated by Harold by that time (narrowly, I would add.) Harold also made an extremely worrying alliance with Phillip Capet of France, my liege! Thankfully nothing seemed to come of it. (no longer possible since 1.6 patch)
Finally, and this is probably as important as the considerations about being on the offensive, you are fighting at least two opponents, not just Harold. And if Svend gets involved or inconvenient alliances are made against you, up to and including the Holy Roman Emperor, you are going to be fighting more. Pray Heinrich stays at home, and be ready to defeat not just your initial target, but any challengers who come after.
Three Phases of Warfare
There are three general phases of the conquest that you can divide your efforts into; these can be applied to your future offensives as well, but they apply particularly strongly for the massive acquisition of land you are making here. These phases are as follows.
-->The "Hot" phase, where you're meeting Harold and Hardradas armies on the field and battling them directly, losing large numbers of soldiers on both sides... or, if you're smart, getting Harold and Hardrada to whittle each other down while you take territory.
-->The "Cold" phase, where you're besieging settlements while at the same time squashing any attempt by either opponent (or a new challenger) to raise fresh armies and intervene.
-->And finally the "Post-Conquest" phase; great, you've got your greedy little mitts on the English crown, but can you hold it? Historically William had to fight fiercely to keep hold of his ill-gotten crown after he seized it from Godwinson; Anglo-Saxon insurgencies only stopped when he basically threatened villages with slaughter if a crown agent was so much as murdered in the area! To add to this, he fought with his own sons, who in typical Medieval grace rebelled against their father over their inheritence. Godwinson could at least take comfort that while William might have taken the throne, he didn't get much opportunity to sit back and enjoy his new powerbase.
The Hot Phase.
Before setting off, consider the following principle. You have a lot of sieges to conduct, and have many armies to confront even past the initial battles. If you keep your army at its present size, these sieges will end quickly and future victories will come cheaply. However, if you bleed soldiers out in battle, your job becomes far harder.
Therefore, don't go looking for Harold or Hardrada; you want to delay your first battle for as long as possible. Harold will begin by attacking Hardrada at York; this is the battle of Stamford Bridge, and Harold will usually win it (I say usually because I have seen some surprising reversals of fortune for Harold during this fight.) This means that straight off, your opponents will be severely weakened.
Don't get complacent. Avoid trailing north to confront the two; let them go the rounds. They will proceed to destroy much of each others power. Take your army across in bitesize chunks (6000 max per boat) to avoid attrition, to Middlesex, and begin to occupy it. Once you've occupied it, work your way north to Essex (NOT Bedford, it only has a supply rating of 10k troops. Don't waste your mens lives!)
With any luck, Harold and Harald will have reduced each other to a relatively small number of men (I lucked out in my second campaign as Harold had only 1500 remaining; this was extremely lucky.) If you've got a 5000 man stack bearing down on you, don't let it get to the occupied Middlesex; engage it where the ground is favourable. 5000 men is a threat to you, and you do not want to confront it on bad ground. So this is one of the few times in this guide where I advise aggression.Take Harold out before he takes up position on that damnable river.
If, however, you have 1500 wretches staggering down from a northern meatgrinder, just wait until they bother your siege at Middlesex, before swooping in to crush them at a time of your choosing. (Before they actually retake anything; garrisons are important assets.) Once you're pretty sure Harold, Hardrada, and any rascals called in from elsewhere are short an army to fight you with, the hot phase can be considered concluded.
The Cold Phase.
I'd recommend sending your spymaster off to establish a spy ring in Trondelag, the Norwegian kings quarters. This is because you start off with an unfavourable chance of assassinating the guy, and you want to kill him off to prevent having to fight him later. If Harald dies, your problems die with him.
However, you do not want to take him out just yet. As you methodically besiege settlements, taking regions gradually off Harold, both parties will try to rebuild their forces; they will regularly bother you with stacks of a thousand landing or marching down from the north; you'll be too preoccupied with sieges to prevent Harold from gathering forces unless you luck out. However, you may not have to fight many of these armies; Harold and Harald have an interesting habit of killing one another off rather than bothering you; this is because one new army is usually larger than the other, and your army is unchallengeable. The AI will therefore rather dumbly attack the army it can do something about, leaving your superstack rampaging around England taking region after region. The key principle here is to keep what you have while proceeding at a reasonable pace. Take provinces methodically so that you don't find yourself sprinting all over the kingdom undoing sieges. Don't let either opponent take anything off you (and try to avoid letting Harald get any footholds; in the off-chance the assassination ends badly you will not want him having a foothold on your lands when you confront him for the last time.)
Once you've achieved about an 85% warscore, assuming your army is still intact, you should have recieved a bonus to your assassination chance against Harald. Your chances of success will be above discovery; go for it and keep trying till the great Norwegian king is dead (and hopefully before lightning strikes and you get found out; this didn't happen to me but getting the attempted murder debuff to relations can't be good.) With Harald out of the picture, you can now begin assaults. Harold will go down quickly now assuming your army is in good shape, and the war should finally wrap up in 1071.
Please send William home if he's still on the field by this point. By sheer bad luck, I got the "incapable" event during the last battle of the war. Having come this far, you want your leader to be able to enjoy the spoils of his successful conquest.
With Harolds surrender and Haralds death, hopefully the war will conclude... though you may still have Svend to deal with... bah, he's a pansy anyway.
Before you make peace with Harold, you may want to exploit being a French vassal by disbanding your army and declaring war on the Count of Vexin using the ducal claim you have. You may also want to usurp the Dutchy of Anjou and take the county of Anjou. By doing this, you avoid having to fight the rest of France.
The Post Conquest.
If you've conquered at least one holding in every county before accepting Harold's surrender, you will receive every single holding in England as part of the peace settlement. (If you didn't, you will have Saxon lords in some of the counties and they will have a lower opinion of you and are difficult to remove; furthermore, you will still be at war with Norway and any allies they have dragged into this. Best to go back to your last save before the surrender and grab every county). Keep the game paused.
If your succession law is gavelkind, you will want it changed as soon as possible - which is at least ten years hence. Change your crown law to Medium now, before you create more vassals, to allow you to someday change your succession law to something a little more accomodating to your dynasty - either Seniority or (if you take the advice below and make all your dukes your kinsmen), Elective.
If your ambition was to be exalted among men, create duchies until you get to 1000 prestige. Create only those duchies you plan on giving away - Lancaster, Hereford, Norfolk, Cornwall, etc. You don't want to create a duchy that you would be holding onto yourself as (a) that is a waste of money right now; and (b) you don't want to find yourself holding onto more than two duchies due to the penalty to vassal opinion of you. Unpause for a day, reap the reward of being exalted, and change your ambition to being paragon of virtue. Go through each of the counties and create vassals for all of your churches and cities. Since each church vassalized gives you 25 piety, you will easily top 500 piety and get the reward for that ambition (+100 piety, +1 Learning). Unpause for a day, collect reward, re-pause.
Identify the counties you will want to keep for yourself. Rouen is a no-brainer since it is your only money-maker for the first five years due to the recently-conquered penalty in English counties; beyond that, consider Middlesex, Winchester, Kent, York, Oxford, and other counties that have or allow more than three holdings.
All other counties you will want to give away to Norman nobles. However, you want all of your dukes to be your kinsmen - so bear that in mind as you give counties away. Within your own court you have a couple of kinsmen but if you go to the character list and look at the men in your own realm you will see that you have a few kinsmen in the county of Eu as well. Give titles to those kinsmen who are not already titled or heirs to another title (and remember that as you give counties away you may also create a new heir here and there!). You do not want an heir to another title getting a county or duchy from you as when they inherit the other title they will gain more strength than you want them to have (see article on Keeping Your Vassals Weak).
After giving away enough titles to get you to your demesne limit, open the diplomacy screen for King Philippe of France and request excommunication. Because of your high piety the Pope should agree to it. If you don't excommunicate Philippe, you are inviting an attack from a strong French army (because Philippe has a strong claim on the English crown); and since you get no troops from English counties until they recover from the recently-conquered penalty, you cannot defeat them for a little while. Excommunication will fracture the French kingdom and keep them putting out their own fires instead of attacking you. (Fracturing the French kingdom also pays dividends if you are looking to take Vexin anytime soon, which is de jure part of the duchy of Normandy, or usurp the duchy of Anjou, which you can do by virtue of having the county of Maine in your realm) After putting down a couple of insurrections Philippe will repent his sins and be reconciled to the church, but by the time he does you will have strengthened your forces.
Check how much money you have. You want to keep some gold in reserve in case you have to hire mercenaries in a pinch - maybe 100 gold. Anything beyond that, start buying upgrades. Focus on Rouen for now, as that is the only county making you any money, and get your tax income increased with castle village and castle city. When five years have passed, you will also upgrade your tax income in Middlesex and other English counties.
Before unpausing, check your council. After creating all those vassals, you may have found a new noble who has higher attributes. You don't want to anger any of your dukes, but if you have an untitled noble (or a bishop or mayor) filling a role in your council that can be improved, go ahead and replace him out.
Unpause. You might want to crank up game speed because for the first five years after conquest you have neither the gold nor the troops to do anything substantial. If you have plans for future conquest, you can use this time by having your chancellor fabricate claims in areas you are planning to conquer (Wales? Scotland? Brittany? France?).
After five years, your income will go up by a multiple of five or more and your English counties begin recruiting troops for their levies. By the time your English levies are maxed out you are ready to take on your next conquest.
Contingencies and Concerns.
(This is a section for adding advice regarding things which may go awry from the guide; things like the Holy Roman Empire showing up on your doorstep, Hardrada surviving, Svend declaring war or problems in France occurring, to give examples.)
One point I would make is to ignore any ongoing wars in France that Phillip Capet drags you into. He'll regularly start a war of religious defence over Sardinia. Ignore it; France can handle itself against Kabylia, and even if it doesn't, it's Capets prestige; what do you care? You've got a throne to take and you won't be answerable to the French king for much longer anyway. You're the freaking conqueror; you don't care about some pointless conflict in the backwaters of the mediterranean. Focus on England; Capet isn't going to be an issue for you much longer.