Instructions
Place a reduced portion of your cat's food on the opposite side of the pet door at feeding time. For example, if your cat is outside, place the food on the inside of the door.
Hold the pet door all the way open by pulling the door toward you, and call your cat to you. Show it the food to interest it, so it will hop through the door. Allow your cat to eat the reduced portion of food, and then move to the other side of the door, without the cat.
Place a bit more food in your pet's dish and hold the pet door open slightly less than you did in step 2, presenting the food to your cat to lure it back through the pet door. Allow your cat to eat the food.
Move to the other side of the door and, once again, and lower the door a little more, while enticing your cat by showing it the food. Allow your cat to eat. Move to the opposite side of the door, and leave the flap closed and call your cat. If it doesn't hop through the pet door, open it a little bit toward you and call it again.
Remove the pet door temporarily, if possible, or tape it open if your cat shows signs of resistance. Allow it to become used to the door. If you don't want to leave the door open, continue to work with your cat as described in steps 1 through 4.
How to Train a Cat to Use a Pet Door
Installing a pet door in your home gives cats the freedom to go in and out as they please. Your cat, however, may not automatically begin using the door due to its apprehension. Help your cat discover the cat door is a gateway to the world outside, and a way to return back indoors at its leisure. Invest a bit of time into aiding your cat's comfort level with the pet door, and it soon will be using the door on its own.