Yes, cat flaps are easy to install. They can be a direct replacement for doors that already have a cat flap. They can also be installed in glass doors, UPVC and wooden doors.
You’ll need the help of a glazier for glass or patio doors though. Either to cut the glass safely or replace the lower pane with one that has a hole. UPVC, wood and other types of doors can be cut using simple tools.
The installation is just a matter of placing either side of the cat flap on either side of the door, installing any weatherproofing and securing the two sides together. That’s all there is to it.