My guide for Brytenwalda.
Best to start near the faction of Bernaccia, will tell you why later on.
The first thing I do is go to the villages and buying cattle for less than 300 gold. I also try to recruit if any would care to join me. Buying all the cattle I could get, I set out for any of the towns of Bernaccia mainly Loidis and Eoforwick or whatever its called. I slaughter my cattles and sell the hides for 280 gold or more. If its below 280, I guess its fine since you already get a lot of profit from selling all the beef and hides. I find this to be the easiest way to get rich.
Looking out for bandits and attacking those that have prisoners. Buy cattle and recruit in villages as much as possible. Attack all the bandits that you feel you can win. This will get you renown and experience for your men. Once you get enough gold, try setting up enterprises. I chose making a tannery in Gwent, Dyfed and Brycheniog. Tanneries there would give you 500+ gold (somehow mine is 900). But still choose the one with greater profits.
Once my warband is a little strong, try joining a faction. Starting as a mercenary is better than being a vassal. Since you could leave and join others without the consequence of your liege getting mad at you. Making friends is very important later on in the game. Do tasks for the lords and help them out as much as possible during battles or sieges. From here on, choose the lord that you want to be affiliated with and look for a future wife.
Loot villages and bring the cattle. Slaughter the cattle and bring the hides to your tanneries inventory. This is probably the only enterprise where its easy to supply the raw materials.
Once you're set and your weekly budget will bring you thousands of income instead of expense, join a faction as a vassal. Village is the only fief they would give you, which is okay coz it adds income.
During war, follow the marshal and help him. Make him your friend. Make everybody in the faction your friends. So when the time comes and you friends siege a town, help them. When won ask them if they would support you to get the town. When 5 or more agrees that you should get the town, the king will give it to you. (I ordered my men to join the siege without me, i had only 70 men. But with my allies it was 1k. No effort whatsoever)
Now you have your own town which will make you even richer and will help give you more men. Tell the constable to recruit 50 people. Do this 4 times, you get 200 recruits. Do all of the above for awhile and tell your companions to spread the word that you should be the rightful king.
I advise to not try and get your own kingdom unless you can get 300+ troops in your warband with a lot of heavy infantry and 150k+ gold. This will support your expenses while looking for a town to conquer.
My guide ends here coz I havent gone to the part of making my own kingdom yet.
Be loyal to your liege for the time being and make friends as much as possible so they will join you once you get your own kingdom.