https://theflatearth.win/wiki/....Post:Cat_Flap_Double
Introduce your cat to the flap prior to installation so that they be familiar with the dimensions and shape. Unlock the flap and let your cat explore it from every angle. Make sure the cut-out is level and square. Mark the screw holes and align them with the cat flap opening. The distance between the edges must be equal. Preparation Installing a cat flap in your home is beneficial for the well-being and health of your pet. It will allow your pet to move from the inside to the outside, without having allow them to enter or leave