Update: I've received a lot of good comments about the traditional method of wetting your hands before working with the rice to keep it from sticking to your hands. The plastic wrap makes it easy for beginners to form the balls; once you're comfortable with that, you should definitely try the wet hands method for a more natural way of forming the rice balls.
Step 1: Lay out the ingredients
Place a couple tablespoons of the cooked rice in the center of the plastic wrap.
Place your desired fillings on top of the rice. In this example, I used common "Philly" roll ingredients: smoked salmon, cream cheese, and chopped chives (instead of scallions).
Step 2: Top with more rice
It doesn't really matter how much rice you use. Experiment and see what works best for you.