Place a half sheet of nori horizontally in front of you on your mat. Place rice on left third of nori, leaving border of nori all around. Place filling vertically across middle of rice. Fold near corner of nori over to begin folding into cone shape. Continue to roll until cone is formed.
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.