Purchase the following in order to do it properly: shotgun (any kind), smoke bomb, Kevlar and helmet. You will also need plenty of teammates. Play as a terrorist. Command your teammates to follow you. Go directly to the tunnel going to bomb site A. Shoot your opponents immediately. Afterwards, have the bomb carrier immediately plant the bomb. Oncethe bomb is planted, throw a smoke bomb into the wooden door. Stand beside the door and make sure you cannot be seen by your opponents. Use your shotgun to shoot opponents that come inside the door.