The best way to learn is as a kitten
In most cases then. A cat will have to be at least 6 months old before you can let it go outside independently. It is also important that he has already been vaccinated (and chipped).

Make sure the cat flap is open for the first period
If you do not open the cat flap, chances are that your cat will not realize that he can get through to go outside. So open the cat flap completely so that your cat sees the “hole” in the door. Also give your cat enough time to get used to this. You can open the cat flap by attaching it to the door with tape. Depending on your cat’s character, he may or may not go through it himself. If your cat is curious, he will discover in no time that he can get through it. If your cat is more reserved then you can lure him through the hole in the door with his favourite treats. It is important that you don’t put pressure on your cat and that you give him the time for this.
The cat flap cover can go down after a few days to weeks.
When your cat is used to the hole in the door, the tape can be removed. Lower the cover, regularly tap the cover so that your cat can see how the cover moves. Also reward your cat with treats when it comes close to the flap. Give your cat enough time to get used to it. In any case, never push your cat through the cat flap. This can be very unpleasant for him so that he can eventually even avoid the cat flap.
Show your cat how the cat flap works
You can do this by letting his favorite toy go through the cat flap and leaving it on the other side. The advantage of many cat doors is that the doors are transparent. Your cat will see the toy on the other side. You can also stimulate his curiosity by playing with a toy on the other side of the cat flap. In many cases, your cat will not be able to control its curiosity and want to go through the cat flap. Instead of toys you can of course also use your cat’s favorite treats for this.
It is possible that your cat will start moving the flap of the cat flap with its paw in the beginning. That is a good start, the cat will automatically start using the cat flap.
The different types of cat shutters, which should you choose?
Training a cat to use a cat flap is easiest when the cat flap has a transparent flap. When purchasing a cat flap, also note the size of the flap and the (expected) size of your cat. If your cat weighs more than 7 kg, go for a large cat flap. (And if there is a dog in the house that will also use the cat flap, choose a larger size).
A cat flap that only lets your cat in
Of course you do not want other neighborhood cats to enter your home through the cat flap. If you choose a cat flap with magnet or infrared, the flap will only open if your cat comes close with its collar with magnet or infrared. The advantage only your cat with his collar can enter. The disadvantage is that your cat can lose the magnet or the entire infrared collar so that he can no longer enter the house. You can prevent this by choosing a cat flap with a chip. Because when your cat is chipped, this cat flap can be set so that it only opens if your cat with chip can open it. And of course your cat cannot lose a chip. For example, you can even set one pet to go outside through the hatch and the other can not.