Click on the arrows on the screen to move. Point and click to shoot your gun when the time comes.
Level 1: Click the left arrow. Kill the stick and get the gun. Click the right arrow 2 times and enter the door.
Level 2: Point and click your gun at the sticks and shoot them (Headshots). If you absolutely can't get past this level, theres a cheat, on the direction screen pops up when you retry, click the comma (,) after the first "ok" in the first sentence, and it will take you past the level, but you wont get the "sharpshooter" metal.
Level 3: Click on the ladder. Shoot the ceiling above the riot shield stick to ricochet the bullets, and kill him. Click on the ladder to climb up the ladder to the roof.
Level 4: Click on the stick with the pistol to kill him, then click on the box