Here's a complete one-hour lesson plan including warm-up, instruction, and match play using drills that have been posted here previously.
Warm-up: Figure 8
Drop & Hit
Ken DeHart's drill is a simple and effective way to introduce using a tracking arm when hitting the forehand.
Adapt the original drill and focus only on forehands for the player working with the coach.
Using quick-moving lines or a forehand caterpillar, all players take turns hitting forehands (fed by coach) using the tracking arm and shoulder turn. For every forehand that lands in playable area, players earn a point. As a group they are trying to earn X number of points. Forehands not using the tracking arm do not count toward the total. Extend this activity by repeating, having each individual player trying to be the first to earn the required number of points.
Coach observes match play, encouraging/reminding/reinforcing use of tracking hand on all forehands.