If you have the need to install a dog door you don’t always have to cut a hole through your wall. You may be able to cut a hole through a side door.
Some homeowners will install the door in a garage door and then out the side door.
Just be aware that cutting a hole in your garage door compromises the fire rating of it. Some people don’t care, but when you go to sell the house, it will likely be caught by an inspection and you will have some choices to make.
If you plan on installing a dog door through the door anyway, it should take you about 30 minutes. The pet doors come with a template and you can trace the pattern on the door.
I would drill a hole in each corner of the pattern so that the saw blade has some relief in the cut. Use a jigsaw with the correct blade. It’s a good idea to cover the shoe of the jigsaw with something that will prevent the shoe from scratching the door as you make the cut. Blue painter’s tape seems to work pretty well.
Start in one corner and just connect the holes that you have drilled. When the hole is made, you can install the door halves and just screw it together.
Weatherproof Your Dog Door
Since a dog door cut through your wall is an opening into your house, it should be as weatherproof as you can make it. For this reason, I like the store-bought dog doors that have 2 flaps on them. These have a flap on the interior of the wall and the exterior of the wall.
Cutting a hole in your wall is a big deal, and so you want to make sure that the outside actually stays outside. I’ve seen people make their own dog doors, and they really look like they are homemade (remember too that this is a potential entrance point for a burglar).
Store-bought dog doors get assembled in halves. There is a wall kit that you can buy (usually sold as an addition to the dog door) that bridges the gap between the two halves. This kit keeps all of the wall insulation in the wall and makes for a flawless installation. The two halves mate with the wall kit and then you install long screws in each of the corners to hold it all together.
I would suggest running a bead of caulking on the exterior of the dog door where it meets the wall. For a smooth surface wall, like siding, you can get away with a lesser amount of caulking. For rough surfaces, like stucco, use a thick bead of caulking and then wet your finger and push it into the crevices to fill the gaps.