Put the rice into a large mixing bowl and cover it with cold water. Stir the rice with your hands, massaging it to release the starch. Strain the rice through a sieve and put it back into the bowl. Swirl the rice thoroughly and strain it again. Repeat once more. Now your rice is ready for cooking.
Put the rice into a rice cooker or large saucepan. Add the water and kombu. If using a rice cooker, turn it on. If using a saucepan, cover tightly and set over high heat. When the water boils, reduce heat to very low. Let cook until all the water is absorbed. Turn off heat and let sit, covered, for 15 minutes. Remove the kombu from the cooked rice.
While the rice is cooking, make the dressing. Put the vinegar into a medium saucepan and set it over low heat. Stir in the sugar and salt until dissolved. Remove from heat and let cool.
Spoon the hot rice onto a sheet pan and immediately drizzle the dressing over it. Use a rice paddle or wooden spoon to gently stir the rice to coat it. Be careful not to mash or break the rice.
Let the rice cool until it is just barely warm to the touch. Put the rice into a bowl and cover with a damp towel to keep it moist.
To refresh leftover rice that has been refrigerated, sprinkle it with water, cover with a damp towel and microwave until hot and tender.