Go to the corner where El Rapido is on Infanta. Up the street towards the university, right next to El Rapido you should see a sign that reads Pizza Celina. On the other side of the street by the garden there will probably be quite a few people standing waiting for pizza or yelling up. Go over to where they are standing and when the woman on the third floor pokes her head out yell what you want. "Uno de queso," "uno de jamon," "uno de cebolla," which ever one you want. The menu is on the street door. She will acknowledge the order. Although this may seem very chaotic, Celina is actually very organized and efficient in keeping her orders straight. Watch what the other people do. When your pizza is ready she will yell your order and point to you and lower a basket. Cross the street and take the pizza out of the basket and place your money in. You will know how much it is by the menu. The prices are all in Moneda Nacional so you will either have pesos or you can pay with one dollar bill (street rate 25 pesos/dollar). A pizza without meat averages about 10 pesos, or 40 cents. Queso Especial means extra cheese. Super Especial, which is 25 pesos, has everything--it is chock full. Yell "sube," and she will raise it and then lower it again with your change. Once you get it, you can either eat it in the street or go to El Rapido and buy a drink and eat at their tables. You should fold it and eat it like a taco, taking care not not spill grease on yourself. Peso pizza is a sort of culinary gem in Havana, at least for me. It is cheap and good--unfortunately, cheese and bread is a tiring combination. But Pizza Celina makes it best and getting it is an adventure in itself.
If you are having trouble getting the system, there is usually a foreign student around who is ordering as well, and who has had a lot more experience with Celina--I am sure he/she would help you out. The Cubans will also help you out, but they will probably laugh at you too, they laughed at me a lot, even once I kind of knew what I was doing.