Use 1 cup of hot (or warm) cooked Japanese short grain rice per serving. Add the rice to a small bowl (like a cereal bowl) just big enough to hold it. Jiggle the bowl, moving it in a flat, circular motion, like the motion of a hula hoop, until the rice forms into a ball on its own—a neat kitchen trick Tadashi's mom taught him. This motion packs the rice so it holds together when it grills.
Wet your hands and place the ball of rice between your cupped palms. Now squeeze, flip, and turn the rice ball several times to form it into a triangular shape. This motion takes a little practice, but after a few yaki onigiri, you'll get the hang of it. Make sure not to compact the rice too tight; you want it to just stick together.
Grill yaki onigiri over medium heat. If the fire's too hot, the rice will burn. We like to place the yaki onigiri along the cooler edges of a grill while other foods cook in the hotter center. Watch the rice carefully while it grills; perfect yaki onigiri need constant attention.