A variety of sizes and styles are available to choose from so you should be able to find one that's right for your pet.
This one I did in Glenhaven recently was for a large dog so I used the large door.
Although removing the door isn't crucial, I prefer to do the work with the door off because it simply makes the job easier and neater.
Drilling a hole in one inside corner of the marked area is the next step.
The hole can be any size as long as a jigsaw blade can fit inside with some play.
Next use your jigsaw with a wood blade to cut right along the lines marked earlier with the template and push out the middle section so that you have the opening for the front and back of the cat flap to slot comfortably into.
I always use a primer on the inside of the cut and all the way around on both sides, the last thing you want is for some moisture to get into the gaps and ruin the door.
After a bit of fiddling around, the outside face of your dog door should be screwed to the external door face making sure it is in the middle and square to the edge.
Now all you need to do is to secure the inside component the same way and hang the door and your job is done.
If the pieces don't want to slot together without using force you might have to just trim the opening a fraction so that there is enough movement to join the front and back together.
Now comes the hard part... Train your pet to actually use the new door!