Step 1: Cook your white rice depending on the directions in the manual of your rice cooker or by cooking it on the stove. While the rice is cooking, slice the tuna and avocado. Once rice is done, add rice vinegar and sugar and mix. Lay a sheet of sushi nori on the sushi mat. Spread the rice evenly on the rough side of sushi nori sheet. Do not add too much rice, you want enough rice to just cover the sheet of nori.