To earn this achievement, you must engage in PvP. You have to either be killed by a player and then place a bounty on their head in revenge, and then kill that player, or kill a player that someone else has a bounty on.
To engage in PvP in the normal mode, shoot a friendly player. If they shoot back, then your PvP turns on and you can kill each other. There is also a PvP server that you can get this on fairly easily.
To become Wanted, you can go to someone's base and pick a lock. It'll put a 10 cap bounty on your head, and people might try to kill you, speeding up this process.
There is an event called Monster Mash that pops up occasionally. In this, you can kill the player with the mask without penalty. I'm not able to confirm because I already unlocked it, but if you are playing with friends this might pop there. If you want to try it with me, send me a message with your user, and I'll add you in game.