You'll need an installer for your cat flap for installing an animal door on your uPVC or glass panel. They can also help you with more sophisticated models, like microchip pet flaps. DIY attempts to install a pet flap can often result in issues like draughts and low energy efficiency. https://www.repairmywindowsanddoors.co.uk/letchworth-cat-flap-installer-near-me/