First of all you need to get treats from local pet sores. You can get chicken or meat flavoured treats. Your aim should be that your puppy should earn more treats instead of the task of going through the doggie door.
Train your puppy with help of a friend or a family member as training with two people is idle. However, one person can also train but it will require more focus and time. First you should close all the doors of your house and then persuade your puppy to go through the pet door. You can start from inside or even from outside. The treat will come handy now as you will offer the treat near the door. After your puppy eats and want more than show him the way through doggie door towards more treats. The puppy might be reluctant in passing after the plastic flap hits his back.
In case two people are up for training then one can stay in and the other outside the house. Both should call the puppy for treats and persuade and admire the puppy after he passes through the puppy door.
You should always give your puppy positive feedback and praise him whenever he passes through the door. Remember not to get frustrated during this training.
You may need to give a gentle push to your puppy in order for him to pass through the doggie door and get the treat. Force your puppy to pass the door even if it has reverse effects.