Do you want to introduce your indoor cat to the outdoors? Here's how you can teach an indoor cat to go outside...
Yes, indoor cats can become outdoor cats — if you mean by this that the cat can go outdoors, as well as indoors.
The best way to introduce this is to get the cat used to the garden, by going out there yourself with him. Sit outside while he explores and make the time pleasant with some cat treats. Do this frequently and slowly and let him take his time and gain confidence. Some cats may initially be scared of being allowed outdoors and a few cats may never really take to it, if there are a lot of neighbourhood cats intruding into their territory.
If you are going to install a cat flap, a microchip operated one will keep out neighbouring cats. You may need to teach your cat use a cat flap. The easiest way to do this is to practise at home, before installing the flap, using cat treats to lure him through. Never ever stuff him through the flap — that will put him off the flap altogether. Once the flap is installed, you may need to use something like sticky tape to hold it open, changing the tape from one side to another.