All you really need to do is block the door open an inch or so - so that a little "outside air" gets through, and your owners' proclivity for sticking their little noses in small holes will soon teach them to go in and out. If you make sure they have to "push" the door a little on the way through, they won't have any problem when you take the "block" out. That is, unless they're so fat and lazy they've lost all their "native curiosity."

It is sometimes suggested that you put their litter box near (inside) the door, so that they'll "discover" the door when they go to the box, and associate going in and out with their "littering;" but I'm not sure they go that far (in the human plane of thinking) with their abstract thoughts.

John