In pairs, the players stand on the service line facing each other. Player 1 has two balls.
Player 1 then throws a ball to his/her partner. Player 2 has to return the ball using an underhand pass to player 1 who has shuffled sideways toward the net.
Both players repeat this action until they reach the net, whereby they continue the action back to the service line.
Make sure that all digged balls land in player 1's hand, this way the player returning the ball will develop his/her accuracy.
As the player progress ask them to increase the speed they shuffle or tell player 2 to pass the ball to the player 1's empty hand.
New hitters rarely miss because of their arm. They miss because of their feet. Build a reliable approach, then time it with your setter.
When your hitter gets stuffed, five teammates standing and watching is how you lose the point. Good attack coverage turns a blocked ball into a second swing.
Coaches spend hours correcting arms in serve receive when the real fault is almost always the feet. Fix the movement and the platform fixes itself.