Rinse sushi rice under cold water until clear, then cook for about 20 minutes. Mix in rice vinegar, sugar, and salt while warm.

Slice cucumber and avocado into thin strips. Cut shrimp or crab into manageable pieces.

Lay a bamboo sushi mat on a clean surface, cover with plastic wrap. Place a nori sheet shiny side down.

With damp hands, grab about 1 cup of sushi rice and spread it over two-thirds of the nori sheet.

Layer fillings on the center of the rice-covered nori. Drizzle with sriracha sauce if desired.

Starting from the edge closest to you, roll the nori and rice over the fillings using the mat.

Slice the roll into eight equal pieces with a sharp knife dipped in water.