Using your rice cooker, cook the two cups of rice.
2 cups uncooked Japanese short grain rice, water
When rice is almost finished, add the can of tuna to a small pan, and place over medium heat.
1 can solid white albacore tuna
In a small bowl, combine sugar, soy sauce, and mirin, and pour over the tuna.
1 T soy sauce, 1 T granulated sugar, 1 T mirin
Cook and stir until the sauce is fully absorbed by the tuna.
Spread out a square of plastic wrap, and put some salt on it.
Place a scoop of rice onto the wrap (according to how big you want it, usually around 1/3 of a cup), and spread it out a bit.
Add some tuna and top with a little dollop of mayonnaise. See the video above, but you can mostly just do this to taste and preference.
Wrap the rice up so that it completely surrounds the fillings, and using your hands, shape it into a tight triangle. Again, see video above for demonstration.
When it's shaped and the rice is no longer loose (you should be able to unwrap and it will completely stay together).
Unwrap it and wrap a piece of seaweed around one side. Seaweed is optional, and you can also put ground up sesame seeds on it, or keep it plain!