Players have a ball each and make an attacking approach with the ball in hand.
As they reach the net they should put the ball behind their head, jump and throw the ball over the net into the opposition court.
As they do this they should maintain a good posture and let their arms follow through. Look for good height and ensure the ball lands inside.
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.