Instructions
Use the mouse to aim your targeting reticule. Your enemies are moving around and trying to shoot you, but they're all the same height.
Aim the targeting reticule above the shoulders of your enemies to increase your chances for a headshot. Look for cover in an area where your back is against a wall, then alternate between pressing "A" and "D" to strafe right and left in and out of cover.
Click the left mouse button to fire your weapon when you see the head of an enemy. If your gun is aimed at the correct angle, you either miss your target or kill it with a headshot.