If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
The ingredients starting with pizza base will be highlighted. Click on them in that order,and then click on the box. Click on the pizza again to put in it in the box. The ingredients will vary for the next pizza. You have to make 10 pizzas in 3 minutes in the first level. The number of pizzas to be will increase in the subsequent levels,and limit will also increase.