Preheat the oven. One hour before baking, place a pizza stone in the middle of the oven and preheat to 500F. The goal is for the underside of the crust to be crisp and browned at the same time that the topping ingredients are cooked. If the pizzas are done on top but not browned enough underneath, next time move the stone to a lower position. And if they are getting too dark on the bottom before the tops are finished, move the stone to a higher rack.
Shape the crusts. Sprinkle semolina (or flour) over a 12-inch round piece of parchment, leaving the outer portion of the circle bare. (Baking on parchment helps keep the oven free of burnt semolina and dripped toppings.)
Keep the crust not being working on covered. Place a dough ball seam side down in the semolina, and with oiled fingertips tap the dough down to form a disc. Gently stretch the dough into shape, leaving a thicker rim at the edge and focusing on stretching rather than pressing down. This can be done either by stretching with oiled fingers from the top of the dough, or by sliding the underside of the dough over the backs of floured hands and stretching gently from side to side. When finished, cover the shaped crust and move to the other pizza.