Determine the location and size:
Select the door where you want to place it. This is normally the door which you have to get up most frequently to open. This maybe the kitchen door leading out to the backyard or the garage door. The size of the door depends on the pet that you have. Cats of course need smaller doors while dogs require a larger one. Buy a pet door that is at least two inches larger than the widest part of your pet's body.
Buy a pet door:
Get a pet door of the required size from a hardware or home improvement store. The kit usually includes door, hinges, frames, a cutting template and instructions. These things make installation a lot easier.
The tool kit comes with two frames; outer and inner. Attach the outer pet frame to the exterior side of the door using the given screws. Do the same for the interior one. The interior one will have the plastic flap. You will need a partner's help to hold the frame while you attach the bolts and screws. All done. Make your pet pass through the door and check for any loopholes.
The flap will remain closed when the pet door is not being used and will move easily when your pet is crossing the door.