Install a cat flap, but firstly consider the cat. Pet doors come in several sizes, so make sure the door is not too small for the cat to use. If the cat is not an adult, choose a door that the cat will not outgrow. Most large pet doors can accommodate a pet of up to 30 pounds. If there are larger pets in the house with restricted outside or room to room access, the pet door should be large enough only for the cat.
Pet doors may have manual, magnetic or infrared locks. Locks can be helpful when restricting the cat’s movement. Magnetic and infrared locks come with a key that can be attached to the cat’s collar. Sensors or magnets in the pet door unlock only for the pet wearing the collar key. This prevents strays from entering the home.
Establish the height of cat flap by measuring from the floor to the lowest point of the cat's belly while the cat is standing. The standard height for adult cats is about 5 inches.
Line up the center of the door with that of the cat flap at the previously determined height. Mark corners with a pencil. Note: Some cat flaps come with a template. Substitute this for the cat flap, and trace the whole template instead of only marking the corners.
Fit the inner frame -- the side with the lock and/or flap --- into the hole, inside the door.