Make friends, start a civil war. Defeat the king, behead it. Sounds right to me. But in theory, you can do that creating your own kingdom....
Step 1. Join a faction.
Step 2. Make friends.
Step 3. Desert and make your own kingdom.
Step 4. Convert your friends to your kingdom.
Step 5. Wipe whats left of the old kingdom.
Step 6. Rename your kingdom.