Step 5: Do It To It
You know how to kick, you know where to aim, but the next thing is to know your limitations. If a door doesn't break within a couple of attempts, it probably isn't going to. In fact, if done right, a door should break on the first kick (unless it is a particularly strong one). If you can't break it open, don't keep trying. You're just going to hurt yourself badly, and then you'll be in an even worse spot. Be smart and be logical.