Equipment: Short-grain Japanese rice (or risotto rice), soy sauce, non-stick pan, clingfilm/glad wrap, oil (may be optional), rice cooker (optional), mirin (optional), sesame seeds (optional)
More about yaki-onigiri at Momofuku for 2 or else you can Google it, there are plenty of sites out there discussing the various methods available.
Rice must be freshly made and warm so that it adheres and holds its structure when shaped.
Clingfilm is a great cheat, so the rice does not stick to your hands.
Lightly oil the plate, gently flip the onigiri so that it has a slight coating of oil. This will help when grilling the onigiri.
Grill lightly over low heat until a crust forms. Flip.
Keep flipping, brushing and grilling until beautifully crispy and golden.