Rinse rice under cold water until it runs clear, then cook according to package directions. Once cooked, transfer to a large bowl, and mix in rice vinegar. Using a flat wooden spoon, fold rice itself, and fan until just above room temperature.
Assemble sushi roll by putting nori seaweed wrapper shiny side down onto bamboo mat with its perforated lines (which help you cut the sushi later on) running up and down. Wet fingers, and put a couple tablespoons of rice on wrapper, leaving a 1-inch space at the top of the seaweed. Next, place 2 slices of avocado on top of rice, a couple pieces of crab, then a few slices of cucumber. Dip fingers into water, and seal the 1-inch space at the top.
Starting with the edge of the nori closest to you, firmly tuck vegetables into the nori and roll it away from your body, using the bamboo wrapper to guide you. Once the sushi is completely wrapped inside the bamboo wrapper, give it a tight squeeze to make sure the sushi is sealed. Remove the bamboo wrapper, and place the sushi, seam side down, on a cutting board. Using a slightly damp, sharp knife, cut the sushi in half, and then each half into another half, and then cut those pieces one more time until you have 8 pieces. Serve with soy sauce, ginger, and wasabi!