I wouldn't start my own kingdom if my Right to Rule would be below 50. Anyway.
The key to success is to get your Trainer skill very high. Don't forget to recruit as much as possible "noble" companions (Rolf, Nizar, Lezalit, Alayen, Matheld) and make them vassals by giving them your villages and towns/castles. Make sure each Lord has only 1 fief, that means you can get more and more vassals.
By the game day of 200, I had ~800 renown (from fighting as a vassal), ~30 level, ~Trainer and Leadership up to 6 or so. I had taken a town and a castle, that gave me 5 companion vassals to begin with.
After you have done that, focus solely on defense. They will be coming at you, and they'll come real hard (~1k troops). Try to get as much as troops possible in the Town. Forget about the castle you just took, as your vassals will still stay in your faction if they have no fiefs. So recruit as much men as possible and pray to god they won't attack you while you're away. Around ~300 men will do the trick nicely, as now you should have 5 vassals sticking in your town all the time which gives extra troops.
Now after a while they will sign a truce with you. That might depend on a few factors, though, like how many factions is the enemy at war with. My enemy was Sarranids who were at war with Rhodoks, Khergits and Free Republic (my faction). Sarranids had no towns left (Rhodoks took two, Khergits took one and I took one) and signed truce with Rhodoks and Free Republic. They waged war against Khergits and now, without the pressure from other factions, they took their town back.
Good luck.
