The best way to make own kindom is to have a wife, have some fiefs(castles, starting village), have good relations with lords, especially of your faction, have many manufactures, full garrisons of shooters and infantry with reserves and a ready to go elite cavalry army - full size you can get with you for hard times, when you heavily outnumbered or pinned down. Such army is undestroyable, but hard to refill, and if not garrisoned - horribly expensive. The last one is quite time and money consuming. It's easier with order knights, though there's a big cost in prestige/renown, so the higher your honor, the better.
And the last thing - rebel only after capturing town, your liege usually doesn't want to give it to player.