Mount a bell close to the place where the pet scratches at the door. Several types of bells or chimes will work for training your pet to communicate via bell. Much depends upon your preference and your pet's physical attributes. Small dogs and cats can learn to ring a desk bell sitting on the floor or a small bell attached to the facing or wall next to the door. Larger dogs may utlize a series of Christmas bells affixed to a leather strap hanging from the door knob. Train your dog to nose this bell rather than tap it with his paw to preserve the door's finish.
Don't open the door in your pet's presence unless the bell is rung. When the the animal scratches at the door, take his paw and ring the bell. Open the door immediately after the bell sounds and send the pet outside. Ignore your dog or cat when he scratches to go out. Reward him when he rings the pet's doorbell by opening the door and by giving him feedback such as "Rover, good!"
