Here is the step-by-step installation procedures of properly installing a cat door:
1. First, remove the door from hinges using a hammer and screwdriver.
2. Measure the belly height of your feline friend, then layout the measurement on the outside of your door.
3. Cut the desired cat door opening. You can draw a square or rectangle. The opening should be atleast 2-3 inches above the bottom of the door, and it needs to be about 2 inches larger than your largest cat.
4. Proceed to making a cat door frame. Measure the inside part of the opening. Construct an "L" shaped frame to fit the inside dimensions.
5. Use fasteners to secure frames to the cat door.
6. For an added flair to the cat door, put some colorful paint or finish to it.
7. The cat door installation end with putting a simple mini-curtain as cat barrier. You can use any fabrics for that, even old towels and curtains. Make sure that that the mini-curtain is 2 inches bigger than the opening of he cat door.