In Sniper Elite 3D, you play as a soldier on a fake mission through a city that attackers have taken over.
To move, press WASD. To aim and shoot the weapon, use the mouse and its two buttons and the scope. To run faster, press shift. To jump, press space. To reload ammunition, press R. To crouch, press X.
Slowly but surely make your way through the city and find a good spot to shoot any terrorists you see. Try to get headshots or one-shot kills.
If they see you, they'll shoot back, killing you as well, so don't let your health bar get too low.
Scout the city and kill enemies until you reach the final boss, a man named the Colonel. He will help you stop the invasion and free the hostages.
Have fun and good luck!
