Easy, Inexpensive Doggy Door in a Screen Door

Step 2 Measure

Measure the width of your screen door; cut a piece off the slotted bar that width (with hacksaw or angle grinder, using appropriate eye protection in either case). 

~ ~ ~

Measure the height of the highest part of your dog from the ground.  My tallest dog is 24" from the ground at the top of her head, but she still bumped into the door when it was 26" high.  I'm making this incarnation 30".

~ ~ ~

Measure up from the ground to the height of your door, and mark on both sides.

~ ~ ~

Hold the slotted bar across at that height, and mark where the holes cross the height line.