Pizza stones really are the best way to cook a pizza, but they can take up a lot of space and the good ones aren't always affordable. Use your cast iron skillet as a makeshift pizza stone to still get that crispy crust.
Image from Serious Eats.
You can cook your pizza right in the pan, but flipping the pan over and using the heated underside will crisp up your dough in under two minutes.
Turn your broiler on so it can get warmed up while you heat your pan on the stovetop. Put it on the highest heat possible, and keep it there until you can feel the heat coming through the other side (don't burn yourself!) Then turn it over, put it on your oven rack, and slide your pizza on top. Let it cook for about a minute and a half (and so the toppings are cooked through,) then check to make sure it's done.
This technique works best with thinner crust pizzas. If your cast iron pan is on the smaller side, you'll have to make personal sized pizzas, but that just gives everyone a chance to customise their toppings.
7 Ways to Use a Cast Iron Frying Pan (Besides Frying) [Serious Eats]