Load into the game. Can be done in a random lobby- but is easier with friends.
This achievement is unlocked by killing the final survivor as a traitor; winning the game. Other survivors can leave, or be killed by each other, you only have to kill the last one. You cannot open any cache (the small black supply drops that drop 5 mins in), or open a traitor supply drop (dropped as a reward for traitors after the 8 and 15 minute timers run out for the survivors repairing the objectives. You can, however, use the traitor ability, located below the open crate option- this will not void the achievement, and will add 10% to your tracker for Clandestine.
Simply kill the last player as a traitor, dressed in your original clothes, and you will win the game, popping the achievement.
This achievement is possibly buggy- due to the likelihood of popping for the second player. I’ve been boosting with 7 others since release and have only seen it pop for both players twice, and neither had the achievement already. If it doesn’t pop being the other traitor (the one not killing the final player), simply follow the above in another game and make sure you deliver the final hit.