You can also question the competency of the current king and suggest you would do a better job. Make sure that you have a decent relationship before you broach the subject so as to avoid trouble. If you persuade a lord to defect or you make a companion into a vassal you will need to grant them lands of their own to keep them happy. This will make your faction more powerful but it will also decrease your chances of grabbing land for yourself. You can also improve your chances of victory by recruiting lords to your cause. Making friends will also help because there is a greater chance that other lords will stay neutral when you rebel. Obviously you have to be levelled up, kitted out and in command of a large, well trained army. The problem is keeping your empire and avoiding a crushing attack from the other factions.

In simple terms you simply have to seize a town or city as an independent. We have established the basic process of how to create your own faction. Mount and blade_ warband when should i start my own kingdom

Mount and blade own kingdom