Most important, purchase the correct rice. Sushi is usually made with special Japanese white hulled sushi rice. Everyyday supermarket rice or "converted" rice from the supermarket won't come close and will not bind together properly. So do yourself a favor and spring for the sushi rice.
Measure the rice out (2 cups rice, 2 cups of water). Next, wash the rice a few times until the water runs clear when draining. Transfer the rice to your pot or electric rice cooker and add the measured water. Once cooked, add in 1/3 cup of sushi vinegar. If you don't have it, 1/3 cup of rice vinegar, 2 tbs. sugar, and 1 tsp. salt. (sushi vinegar is packaged seasoned with salt and sugar) Place the rice in a non-reactive bowl (non-metal) and let it cool for a few minutes.
Using a sharp knife, cut the fish into 1/4 inch by 1 inch thin slices. (see above) Place over rolled up rice and serve.
Easy as can be!