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.