Treats! That's how I did it with my three, two of which had been used to going outside but had never used a catflap.
Put kitty outside (or wait til one wants to come in) then fiddle the flap open a bit (towards the inside ... you're inside, hence the 'fiddle' bit) then offer kitty a treat by offering it through the catflap and letting them smell it. Then before they can grab it, let the flap fall shut, still offering the treat where they can see it through the cat flap! It's amazing how quickly they'll learn when treats are involved!
Do this with each one until they consistently get the idea
You may find that once one of them susses it out, the others will anyway. Good luck!