To make the sushi rice:
Cook quinoa and black rice as directed. Once cooked and cooled slightly, mix grains together. While still warm, toss with salt and sugar, then vinegar.
While rice is cooling, make the omelette if you choose to include it. (Whisk the eggs with salt and pepper; pour into a small, lightly greased frying pan over medium heat until egg is set. Flip and cook until egg is cooked through.) Let cool slightly before cutting into strips.
Assembly option #1 - temaki (hand rolls):
Make your own hand rolls: Cut the seaweed sheets into fourths. Take a square of seaweed and top with a spoonful of sushi rice and desired toppings. Roll up, dip into soy sauce, and eat!
Assembly option #2 - maki (rolled) sushi:
Spread a full sheet of seaweed out on a flat surface. Spread rice evenly across the sheet, starting from the bottom and working up to your desired height (you can fill the entire sheet; I usually stopped about 1/3 of the way up for a more equal ratio of rice and filling). Spread rice to the edges that you can't see any seaweed on either side.
Lay toppings in a line parallel to the bottom of the seaweed sheet about 1-2 inches from the bottom. Take the bottom edge of the seaweed and roll it over the toppings, rolling it as tightly as possible. Continue rolling until you reach the end of the seaweed and let the roll rest on the crease where the loose end of the seaweed is—this will help seal the end. Roll the rest of your sushi and use a sharp knife to cut into sections.