Make thin, even slices of the three types of fish. Suggested thickness is 0.5 cm. You can make long and wide slices, and use them to coat the entire roll – or smaller slices to coat only the top of the roll. Cut the cucumber into long thin sticks. It should be as thin as you can, and as long as a roll, or half a roll if the cucumber is too small. The green onions should be chopped into onions or thinner. Cut the nori in half, and cover it with rice. Flip the nori for rolling, and lay the fillings on the nori. Use as much fillings as you want, as long as you can close the sushi roll. Lay the slices on top of the roll so that each topping takes 1/4 of the roll length. For the avocado – Use a vegetable peeler to peel thin slices of the avocado, and lay them directly on the roll to cover the rest of it. Use the avocado immediately after peeling; it will be harder to pick it up after you lay the peels down! Use a bamboo rolling mat wrapped with Saran wrap tape, to tighten the toppings to the roll.