This is self-explanatory. All you have to do is shoot either a civilian, VIP, guard or a target in the head and the trophy will pop straight afterwards. For some you may find it easier to get the Headshot in 1st person mode. You can do this by pressing as it does give you a more accurate shot. This trophy can potentially be missed if you sweep through the game stealthily, however the best chance is in the last mission as you can shoot your target clearly in the head – see the Professional Difficulty, Silent Assassin and Weapons Guide and read Mission 12: Hunter and Hunted.
If you do not get it in your initial playthrough, then go for this trophy when you are going for 47 Kills in a Mission.