Very simple. Pretty early on in the game you get your first EP (prototype's kind of XP for upgrades) you can go to the upgrade screen.
Under the "disguise" tab choose "Patsy".
Check out any area which has some militairy. Press LT and RB button so that a soldier is selected and press X. Whenever you pin point that soldier (you will say something in the line of "LOOK! That's him!") and the soldier gets killed you did it right and it counts.
It takes about a minute for the patsy ability to "recharge".
After you did this five times the achievement will pop.