How do you train a dog to sit at the door when it has to potty?

How do you train a dog to sit at the door or bark when it has to potty?

My mom's dog will nudge you, then if you don't get up right away, she'll potty in the floor.

Answer:

House train the dog. Before letting it go outside to use the bathroom make it sit b4 you open the door. Do this every time and he will pick up that if he sits at the door he can go out to pee.