Unscrew the face ring from the return inlet.
Pull the ball from the inside of the return inlet with your fingers. Bag it and place it near your pool supplies in a spot that will be easy to locate come spring.
Push a rubber winterizing plug firmly into the inlet fitting. The winterizing plug should come with your above-ground pool. If you do not have one, contact your pool's manufacturer. They may send one to you, or recommend a pool supplier where you can get one. If you can't get in touch with the manufacturer, head to a pool supply yourself with measurements of the width of the inlet fitting.