To perform a stealth kill, you press
when prompted on screen. (You must be directly behind an enemy.)
There are two ways you can go about this trophy. You can simply play the entire campaign and slowly work your way towards this (you are given many opportunities to perform a stealth kill) or you can simply reload a checkpoint.
During Act 1 Mission 2 (after you rescue the hostage) you will be directed by your superior to head to a hotel. On your way you will see an enemy with his back to you. You are instructed on screen to stealth kill him. Do so and you can reload checkpoint until you get the trophy.