You can use various methods to teach a cat to use a cat flap, much depends on what motivates your cat. As long as the cat is comfortable in the house, you can use food or fun to entice them out.
Stand outside the door on the other side of the cat flap and call your cat. Use either the smell of a treat or some tuna to entice them out or use their favourite cat toy. Be patient, use an excited voice and let them figure things out themselves.
Don’t be tempted to push the cat through as this can cause a negative association. Cats are bright, give them the motivation and they can figure the rest out themselves!