The first thing you need to do is teach your dog to sit in the correct position.
Sit with your dog in a chair, or on your lap, but not on the ground, because that can be dangerous for your dog.
Sit on the end of a stool or crate, and not on a crate pad or table, because the dog may not have access to a proper seat and may be more easily injured by falling.
A dog that sits on the floor or crate has no natural position and may have to be taught to lie on its back.
Learn how to teach your pet to sit on its hind legs.