http://brewwiki.win/wiki/Post:Fitting_a_Cat_Flap
Install an animal flap if you want your pet to be able to leave and come back anytime they want without opening the front door. You should hire a professional to install the cat flap. They can handle the complexity of putting in different types of cat flaps to various doors and walls. Installation With a cat flap, your pet can come in and leave the home at will, without needing to open the door. The range of pet flaps available on market is vast, including microchip-proof and bite-proof doors to fully lined cat flaps. They are available