Bounty can be earned in many ways:
Breaking into locked house's
Robbing shops while people or the shop keeper can see you
Attacking the town guards and other none violent NPC's with any attack or weapon
Just keep doing this until your bounty is up to 15,000. When you become king and ruler of Albion no one is going to try and arrest you! You also need to stay in one region to make it count. Leaving will void the achievement making it reset to 0 Gold. The best place to get a bounty on your head fast is Bowerstone Castle!
If you haven't done it when you become the ruler then you'll have to get this in another player’s world as your ruler status is ignored when away from your save.