Training should always be based on postive reinforcement - rewarding the cat when it has done what has been asked.

Training a Cat to use a Cat Door

Once the cat door is fitted the cat can be trained to use it by propping the flap open and enticing the cat to go through the open door with some food or a treat. Once the cat is happy going through the fully open flap, it can be propped so that it is only open a little way and enticing the cat through again, gradually lowering the flap a little at a time until the cat realises that it can push the door open and climb through when it is closed.