Take the door off its hinges. This is usually a two person job.
Consider the height of your pet and the height of the gate and mark on the gate where the dog gate will be placed.
Big dogs need big doors. Measure your dog's shoulder height and make sure you choose a dog gate that is large enough to accommodate your dog.
Drill holes in the 4 corners of the door (Picture 1) and cut out the door with a hand held circular saw.
Dry fit the doggie door to your cut hole (Image 2).
Doors with raised panels may require filler pieces to support the door (Image 3).
Screw on the front and back of the doggie door.
Put the large door back on its hinges.
Once the door has been installed, it is time for the final test. Have your dog do a test run. He should fit comfortably through the door.