https://zenwriting.net/catrice....99/installing-a-upvc
A cat flap is a great option to give your pet some freedom and ease the burden of letting them in and out. It will also help your pet to feel less stressed and keep them from entering your home when they're not wanted. To install a cat flap take a measurement of the distance from the bottom of the cat's stomach to the floor. Draw a line along the door's panel. Verify that the line is straight using the spirit level. 1. The kind of door If you're installing the cat flap to the uPVC door panel or a double glazing glas