Make sure your dough has enough gluten development to hold together during. Stretch the dough a bit on the counter before picking it up. Then keep a flat palm and your fingers together. As you slap the pizza back and forth between your hands, rotate your hand 90° to the other hand in order to turn the pizza. Once you get that motion down it will be pretty easy to move to tossing the pizza in the air just after slapping the pizza into your hand.

Source: worked for Papa Johns as a teen.