All you've got to do is go up behind an enemy as if you're about to do a takedown, then pull out your sniper and shoot them.
The easiest place to get this done (for me at least) is the sniper's nest in Mining Town located approximately at 23.22; 20.69. There is a short path going up to the hill though, with landmines near the top. Using scout mode at the bottom near the road, it'll show you a trail. Just follow that and avoid the landmines. Eventually it'll lead you to the sniper, who's laying down at the edge of the hill.