Rinse the sushi rice under cold water until the water runs clear.

Combine the rinsed rice and water in a rice cooker or pot. Cook according to the rice cooker's instructions or bring to a boil and simmer covered for about 20 minutes.

While the rice is cooking, mix the rice vinegar, sugar, and salt in a small saucepan over low heat until dissolved.

Once the rice is done, fluff it with a fork and let it cool slightly.

Gently fold the vinegar mixture into the warm rice with a wooden spatula.

Lay a sheet of nori shiny side down on a sushi mat. Wet your hands and spread a thin layer of rice over the nori, leaving about an inch at the top edge.

Place your desired fillings—slices of fish, cucumber, and avocado—across the rice.

Using the mat, roll the sushi tightly away from you, applying gentle pressure. Once rolled, use a sharp knife to slice into bite-sized pieces.