Serves: 4 rolls
For the rice:
1) Mix together vinegar, sugar, and salt.
2) Rinse the rice with cool water in a colander and drain. Put the rice and water in a medium saucepan with a tight fitting lid. Bring to a boil over high heat, then reduce heat to very low and simmer the rice, tightly covered. (For a good seal, wrap the pan's lid tightly with a small kitchen towel and cover the saucepan. Make sure the towel's edges are folded up well away from the heat.) Cook the rice until all the water is absorbed, about 15 minutes. Remove the rice from the heat (do not uncover), and set aside for 10 minutes.
3) Fluff the rice with a fork and transfer to a large bowl. Add the vinegar mixture and toss with a wooden spoon. Spread the rice out on a parchment paper-lined baking sheet. Fan the rice continuously with a fan or magazine until cooled. Cover the rice with a damp towel.
To assemble rolls:
1) Place the sushi mat with the slats running crosswise directly in front of you on a clean work space. Lay a sheet of nori shiny-side down and lined up with the closest edge of the mat. Light moisten fingers with water, and evenly press about 1/4 of the rice onto the nori, with 3cm uncovered at the far edge. Line up the fillings about 2cm from the closest near edge, evenly across the rice. Leave 2cm of rice uncovered at the far edge.
2) Beginning with the near edge of the mat, tightly roll up the nori, rice, and fillings into a cylinder. Firmly tug on the rounded mat over the roll as you pull on the far edge of the mat to tighten the roll.
3) Open the mat, dab unsealed edge of nori with a bit of water and roll the sushi forward to seal. Transfer the sushi to a plate and cover with damp paper towels. Repeat with remaining rice and fillings.
4) Cut each sushi roll, crosswise, into 8 pieces with a sharp wet knife. Serve with bowl of soy sauce for dipping.