You can pick this Achievement up pretty early in the game as well as the excellent video method posted by lozzy7. I notched this shortly after acquiring the bow. Unfortunately, I'm not well set-up to put together a video guide, so bear with my descriptions.
Just past the bow, you'll reach a compound called the MG Base. (It's just beyond the area with all the pipes, which you can climb onto and assault from above.) Once you get inside, your AI partner will head out on his own and leave you to progress alone. Here's where you can start tallying your headshots.
In the first room, there are four guards. Hang back and wait for the one behind those crates closest to your right to reveal himself. You should be able to plunk him in the back of the head with your bow (or a well-placed SMG shot - but I recommend the bow) and pick off the other three one-by-one as they move in to investigate. Before you leave this area, climb the ladder on the left to restock on arrows.
In the next room, there are lots of places to lay low while waiting for prey. Follow the first guard around to the right side and headshot him, then jump under the stairs to the right and hide behind all the boxes. This makes a great ambush point, and you can probably get another 4-5 more before you have to move out again. At least wait until the 2-man patrol comes down the stairs over your head. You don't want to get boxed in.
You should only need one or two more by the time you step into the open, so dart out to the far side and go about half-way up the steps to draw attention and lure a couple to the stairs. If you double back down to the ground floor, you can hit them as they come down after you - they tend to only send one at a time. Two might start down the stairs, but one usually turns back about half-way down.