Tab or scroll across to the Contracts menu and select Featured Contract. From here select and complete any of the contracts to earn the achievement.
An easy one to do is the Final Test Contract where you have to kill the two guards that are closest to your starting point, Christopher May and Luca Wood.
When you start hide behind the car to your left (the one used to exit the mission). When The guard walks past the gate and has stopped by the fence, throw a coin towards the boxes on the left. He will walk over to check it out shoot him in the head, then turn to face the guard in the small hut by the gate and shoot him in the head as well. Now press the button by the car to exit the level.
The achievement will pop when you get to the page showing your score.