Organization
This skill can be introduced through a full class demonstration. Ideally this task would be teacher-centered, and the teacher would remain in control of pacing by being the server. However, in a large class the task would be student centered and could be introduced by running the class through a demonstration. Then divide the class into two different groups and have them work on separate nets.
Possible Modifications
Allow players to catch the ball on serve receive, toss to themselves and then pass.
Allow players to toss the ball over the net if they are not ready to learn the underhand serve.