Depending on the layout of your house, a side entrance is usually the best place to install a cat door. However, a front door is not the best choice, as your cat might be vulnerable to predators and other cats. In this case, you should install a cat door on the back door instead. To install a cat door on the back of a solid wood door, you should first measure the opening of the door. Ideally, the opening is at least three inches from the door's edge. You can do this by using a pencil or tape to mark the position of the opening. Once you have the right measurements, cut the cat door. Make sure that the door is the right height for your pet. This is important because cats take time to adjust to a new environment. Therefore, they may not show much interest in the door. Also, you should be sure that the door fits correctly in the hole in the door. In addition to front doors, you can also install a cat flap in a side alley. This will give your cat a more hidden entrance and exit. Just be careful not to install it near elevated perches, because a neighbor's cat may ambush your cat from there. Besides, cats are territorial animals and tend to patrol their territory in a circular fashion. It's best to have more than one cat door in your house, so that your cat can freely move around and enter the house without having to worry about the other pets.