Place a sheet of nori, shiny side down, on a bamboo rolling mat (with slats running parallel to you) or a piece of plastic wrap (about 10 by 12 in.), aligning nori with the bottom edge of mat. Dip your hands in vinegar-water mixture and scatter 1 1/2 cups rice over nori; pat into an even layer, spreading out to the sides and bottom of nori, but leaving a 2-inch-wide strip bare along the top edge.
With your finger, spread a thin stripe of wasabi horizontally across center of rice. Arrange three or four sushi fillings along wasabi stripe; it's okay if some ends stick out). Moisten top edge of nori with vinegar-water mixture. Holding fillings down with your fingers, lift edge of mat closest to you with your thumbs and roll it over so that the near edge of nori meets the far edge of rice. Roll over to seal, and briefly press mat around roll.
Remove roll from mat. If desired, trim off any filling sticking out. With a sharp knife, cut roll into six or eight equal pieces, rinsing knife in water between cuts.
NOTES: Set up a rolling station: Arrange sushi components on the table, or set up a station on a nearby counter. Mix 1/4 cup rice vinegar with 4 cups water. Pour into small bowls for guests to moisten their hands while rolling sushi, so rice doesn't stick. Put 6 precut sheets of nori (7 1/2 by 8 in.) in a covered serving container to keep dry; mound sushi rice and fillings on platters or in bowls (vegetables together, seafood separately). Put condiments--soy sauce, pickled ginger (1 cup), and wasabi (about 3 tablespoons prepared wasabi or 4 tablespoons powdered wasabi mixed with about 2 1/2 tablespoons water)--in separate bowls. Arrange ingredients, along with bamboo mats or plastic wrap, on the table or station.
Nutritional analysis per piece of crab, avocado, and cucumber roll: Calories 133 (15% from fat); Fat 2g (sat 4g); Protein 2g; Carb 25g; Fiber 4g; Chol 2mg; Sodium 162mg.
Nutritional analysis per piece of smoked salmon, asparagus, and carrot roll: Calories 112 (2% from fat); Fat 2g (sat 1g); Protein 3g; Carb 24g; Fiber 4g; Chol 5mg; Sodium 126mg.