The bell hangs on a rope from the rear doorknob. When we first got her, we'd take her to the back door, ring the bell
...and then let her out. Whenever she stood at the door wanting to go out, we'd ring it, then let her out. She figured it out.
The cat watched this happening, and figured it out too. If we had tried to train the cat, it never would have worked. But somehow watching the dog, the cat learned it.