Position (1) receiver at the service line. This person will have serves hit to them from someone standing right on the other side of the net. This is a beginner drill to help kids/beginners get use to the serve, so these serves will be hit softly at first. As the player gets more advanced both the receiver and the server should back up a few steps until both players can serve and return serve from the baselines. If you have (2) kids you can have both stand behind a service box and a serve hit to each kid in rotation.
Be sure to have the server serve to both sides of the receiver(s) so they can work on both their backhand and forehand.