Split each nori sheet in half.
If you have a bamboo mat, lay it down on your workspace. If not, wax paper will do fine. Personally, I like using wax paper on top of a bamboo mat.
Lay one half sheet of nori on your workspace, and carefully spread 1/2 cup of sushi rice over it. Dampening your fingers will help prevent the rice from sticking to your hands. Press rice into nori.
If you are using sesame seeds, sprinkle some over the rice, and lightly press in.
Carefully flip your riced-up nori over.
Lay out a row of tuna slices, followed by mango and avocado slices.
Using your bamboo mat or wax paper, carefully roll your maki tightly, starting from the side with the toppings. Make sure to keep the toppings from spilling out the sides. Also, don’t let your mat/paper get caught in the roll. It may take some practice!
While maki is still encased in the mat / paper, form it into a round log, compressing it slightly.
Unwrap your maki from the paper or mat, and place on a cutting board. I find that a damp serrated knife works best. Anything else, and I make a huge mess of my precious maki!
Slice into approximately 3/4″ – 1″ pieces.