You need negative relations with a faction to conquer its fiefs. If you did not manage to secede from your liege with lands and are a landless unaffiliated person, all that is required is to start conquering. If you are currently questing for a claimant, you can convince the claimant to abandon the rebellion, and you will take over the entire territory you conquered for them at the cost of honor (you will receive the ability to rename your faction and give fiefs to your heroes). If you already are, there are three ways out: ask to be released from your oath, be denied a fief you requested and dispute the judgment by force, or ask your husband to join you in a rebellion ( female characters only).
In other words, you must not be part of a rebellion or a vassal of one of the current factions. The first requirement of ascending to the throne is that you cannot be tied to another claimant. Also, it will give you a fair fight when you start a level 5 king is not going to be popular or effective, at least level 20 is recommended. This gives you a chance to make friends, who will become your vassals. When starting the game, it is best to first be a vassal of another kingdom, and when you are powerful, revolt and start your own kingdom. Before you start your own kingdom, make sure you have a lot of renown, either honor or dishonor (depending on your liking and desired relations), and especially, a lot of right to rule.
