1. Holding the Ball
In your non racket hand let the ball lightly rest in the bottom two segments of your fingers, pad of your hand, and your thumb. Your hand should be positioned as if you are holding a glass of water.
2. Lay Up
First, think in terms of floating the ball instead of tossing. There needs to be finesse rather than abrupt movements. A ball toss could be compared to a lay up similar to basketball. Now, float the ball up as if you were tossing a glass of water and need to catch the water back in the glass. When you release the ball it should be at the point where your arm is straight and about eye level.
3. Toss Height
The toss height should be a little higher than the tip of your racket if you were to reach up and extend your racket. There needs to be enough height margin so you can complete the swing. Please know that too much height in the toss can be harder in terms of timing and control.