Draw your ideas on paper. You probably will want to have other items in the scene besides the couch, like a coffee table and maybe a TV, as well as people.
Decide what the arrangement of the figures in the scene will be. This is where the artistic statement can be made. You might want to have two teens sitting there mesmerized by a video game. Or you could have a father asleep on the couch with a beer in his hand.
Choose the method for creating your diorama. While you could make everything from scratch, it is more customary to use dolls and doll furnishings. This can be from a popular set of girls' dolls, or you could use boys' action figures.
Cut one side out of a cardboard box that is the right proportions for the living room that will contain your couch and its owners. Use spray paint to paint the floor and the walls whatever color you like.
Position the couch to match its position in your drawings. Then place the other furnishings and pose the human or other figures you will be using. Your diorama is complete.