Pretty simple overall. At the beginning of the level "Night Shift" you are instructed to shoot out four hanging lights with the infrared sniper. They are to the lower right when you look over the edge. The lights all sway a little bit, but if you click in the left analog stick to hold your breath and steady your shot, it shouldn't be too hard to hit all four without missing. You DO NOT need to account for any bullet drop from the long distance of the shot, at least I didn't when I attempted this on the "easy" difficulty. Just be sure you are aiming for the physical light fixture, which can be tough to make out because of the infrared scope.
If you miss, just pause and reload the checkpoint and try again.