First and Foremost, you should stop playing as a count, it is no longer as straightforward as it used to be. By playing as a weak starting character, you are intentionally handicapping yourself for a game you were completely unprepared for. Try playing as a french duke to begin with. That means that you will have the power of your liege for defensive wars, and can declare war on people much weaker than you, ie rebellious counts and weaker dukes. You should start out only going to war with people who you outnumber by 50 percent, minimum, since you have to deal with seiging and defended bonuses.
You should try not to rely on warfare. Making the right marriage matches and waiting up to an hour for them to come to fruition is part of the game, (playing the long game is part of all paradox games really). In fact, Marriage is actually a much stronger mechanic for taking territory than combat. Not only that, but marriage is the most powerful mechanic in combat as well, because it allows you to call allies with vastly larger militaries than you can command early in the game. An alliance with the king of NearbyNation will let you call in thousands or tens of thousands of additional troops.
Still, the overall point is to stop playing as weak characters. Counts and the occasional 1 province duke in ireland are not advisable start locations for someone unfamiliar with paradox games, because like real life, paradox combat is very swingy. A 20 percent difference in starting numbers can be a 200 percent difference in casualties. That means when starting out, make sure you absolutely have a substantial numerical advantage against whoever you declare war on.
As for your example of losing a battle where you had superior numbers, you probably made a mistake with regards to combat morale, but its difficult to know without screenshots. Even if they had a vastly better general you should not lose a 1000 to 2500 fight, unless you had virtually no morale (probably from not paying the men).
Basic Concepts to Practice:
Revoking a county title as a Duke. This usually starts a war of rebellion that you can crush.
Invading an independent county as a Duke. This includes ones rebelling from their liege that have become independent temporarily
Marrying daughters into a powerful alliance (nearby, but not where you want to expand to)
Marrying sons to inherit land
Keeping your most important vassals and advisors (particularly the spymaster) happy.
As others have pointed out, essentially nothing. You may or may not be able to call him into wars, and he may or may not do the same.
Interesting thing happens though if you're an emperor-tier ruler and have an anti-pope installed. You gain a CB to install your anti-pope, and if successful can vassalize the pope (assuming you have a de jure claim or the anti-pope is of your dynasty, as usual).
Best Answer
I got CK2 a week back during the Steam sales. I realised that most of the walkthroughs for William the Bastard are no longer valid as they were written back in 2012. As what Paul Marshall had said - the patches had altered the way the AI worked.
Two nights ago I finally managed to become King of England with William (after at many many replays). I think a fair amount of luck is involved plus the fact that I decided to splurge on a Mercenary company (I chose the Great Company for its heavy calavry. )
These are some of the actions I do to maximise my chance of winning.
When I first load up the game. I did my usual marriage and betrothal actions for alliances. Currently I am try to get an alliance with King of Norway by marrying William's half-brother (Odo) to his daughter. Then my sons to Denmark, Hungry or HRE. My experience was that immediately after I became King of England, HRE came after me instead of France and that ended my game rather quickly because there was no way I could win a war with HRE after all my troops were attrited.
Then I send my Spymaster to York immediately so that I can have a sense of the troop movement. Nine times out of ten, Harold wins against Norway without much losses. From there I keep track of Harold and have a sense when his troops woild arrive at Normandy. Usually it would take 2 months from the start of the game.
At the end of the first month, I would hire the merc called Great Company for the hvy calavry as I noticed that Harold does not bring them over to Normandy. What I do then is to wait until Harold lands and seige one of my county. Then I launch an immediate assult at him. The chances of winning is more than even. Thereafter I would keep going after Harold until his troops are depleted before he could flee back to England. This is important because if he gets back with more than 2k troops he is still a threat to you when you start your invasion. As yourself will also be too depleted.
At this point I take a look at my own troops. If I have more than 6k troops after dismissing my mercs, I continue with the game as it would give me sufficient troops to start sieging the minor counties in England. The reason I dismissed the mercs is because I found their costs unstainable and I became bankrupt after a few years if I am too slow in forcing the surrender. At this point I would say luck plays a large part. Because if you are lucky and net a few prisoners that gives 100 to 125 gold then keeping the mercs is not a problem.
I hope these opening moves help. Good Luck!