You can install a pet door while the door is still attached to its frame, but we've taken ours off to make it easier. We've placed it onto a workbench and rested the door on some timber offcuts to avoid cutting the workbench when we use the jigsaw.
Make sure the pet door is at the right height for your pet to get in and out. To do this, measure the distance from the underside of your pet's stomach to the floor. Mark that distance from the bottom of the door at two points so that the pet door is square.
Cut out the pet door template which is supplied in the kit. Line it up with the two marks you've made on the door and trace around it.
Place masking tape around the edges of the template lines you drew on the door. This will help to protect the door when you cut it. Use a cordless drill and the right sized drill bit to drill four holes in each corner of the template that you've marked on the door.
Put on your safety glasses, dust mask and ear muffs. Insert the jigsaw into one of the holes you drilled and cut out the opening for the pet door.
Place both sides of the pet door into the hole on either side of the door. Secure it to the door with the screws provided in the kit.