Place a nori sheet lengthwise on a bamboo rolling mat, shiny-side down. Position the sheet about 1-inch from the edge of the mat closest to you and leave some of the bamboo mat exposed on either side of the nori sheet. Wet your hands in cool water and take a handful of sushi rice. Place the rice in the center of the nori and use your fingers to spread the rice evenly over the nori. Be sure to leave a 3/4-inch strip of nori uncovered on the far side. Place tuna strips and some julienne vegetable, cucumber or avocado along the center of the rice. Be careful not to overfill the nori. Place your fingertips over the fillings to hold them in place. Then, use your thumbs to lift up the edge of the bamboo rolling mat closest to you. Begin rolling the mat away from you, while applying pressure to the fillings to keep the roll firm. Roll the mat over slowly until it covers the rice and the near and far sides of rice join, still leaving the 3/4-inch strip of nori, rice-free, exposed. While holding the bamboo mat in position, apply pressure to the roll with your fingers to make the roll firm. Slice the roll in half, then cut both rolls twice to make 6 equal sized pieces. Repeat this process with the salmon and various fillings, nori and rice.