Get a tennis ball bucket (72 ball bucket) place it in the back corner and serve, The ball must land and stay in the bucket!
Aim for the serve to hit the front wall 1 foot below the outline the ball will continue up and then drop into the bucket.
points to note:
1. you must lift the ball high on the front wall
2. the ball must drop in to the bucket and stay in the bucket
progression: raise the bucket by placing it on a chair!
hope this helps!