You will need a hole in the glass to install these cat flaps. If you’re replacing an old flap, make sure your new flap is the same dimensions as your old one. If you’re adding a new cat flap to a glass door, you’ll need a glazier to cut the hole or replace the glass panel with one with a hole.
When you’re ready to install, place either side of the cat flap over the hole, add any weather seal and use the fixings to secure the cat flap in place. Once done, test the door a few times to ensure free operation and then introduce your cat to their new door.