You must have the technique "A Brief Shadow," which allows you to kill from a hiding place, in order to unlock this achievement. To get the technique, you must play through the sixth mission ("An Ancestral Home") and then spend 6 points on the technique in the Upgrades screen.
Equip yourself with a noisemaker and load the level "The Trail of the Shadow". You'll have two guards to take care of soon after starting the level, then a guard and a dog just after that. Immediately after this you'll climb up and see a lone guard. Don't kill him right away; instead, hook on to the light above his head, then jump off it to the right. You should land outside of his field of vision, on the other side of the dumpster. Jump in the dumpster, then use a noisemaker to draw him close.
Assuming you have the technique available, as soon as the guard gets close enough you should get an X prompt. Try pushing your left joystick toward him if it doesn't appear. When it does, press it, and enjoy your achievement.