Place sheet of nori vertically on mat. Wet fingers and palms with rice vinegar and spread about 1 cup of sushi rice evenly over nori. Cover with plastic wrap and press evenly. Turn over so that now nori is on top and the plastic wrap is on the mat. Lay fillings horizontally across the end of nori closest to you. Roll almost one complete turn, stopping to press and make firm, then complete the roll. Dampen the knife with rice vinegar and cut roll into 1 inch slices. Gently remove the plastic wrap.
Serve with pickled ginger, wasabi and soy sauce as condiments
Rinse rice in water until the water runs clear and then drain in a colander for 1 hour. Place the drained rice in a rice cooker or in a pot with a tight-fitting lid and add 4 cups water. Over medium heat, cover and bring the water to a boil. Boil for about 2 minutes, reduce heat and allow to simmer for another 5 minutes. Reduce heat to low and cook for about 15 minutes, or until water has been absorbed. Remove from the heat, remove lid, and place a towel over pot. Replace lid and let stand for 10 to 15 minutes.
While the rice cooks, combine vinegar, sugar and remaining 1/4 cup water in a saucepan. Heat over low temperatures, stirring, until sugar and salt dissolve. Let cool.
Empty rice into a hangiri (or other nonmetallic) tub and spread it evenly over the bottom with a shamoji (or large wooden spoon). Run the spatula through the rice in slicing motions to separate the grains. While doing this, slowly add vinegar mixture. Add only as much as is necessary; the rice should not be mushy. If you have help, fan the rice with an uchiwa (fan) during the cooling and mixing procedures.
Do not refrigerate the rice. Keep it in the tub covered with a clean cloth until ready to use. The rice will last one day.