How to Make It
In a heavy-bottomed pot, rinse the rice in several changes of cool water, until the water runs clear. Pour off all the rinse water. Add the 3 cups water.
Cover the pot with a tight fitting lid. Do not remove this lid until the rice is finished.
Bring the pot to a full boil. You will hear it rumble, and steam will push its way out at the edges of the lid, and the water may even boil over a little. Turn the burner down to the lowest possible setting, and set a timer for 18 minutes.
While the rice is cooking, make the seasoned vinegar. Combine the sugar and salt with the vinegar in a jar, and shake until they dissolve; set aside.
When the timer goes off, return the heat to high for one minute, or until you hear the remaining moisture boil again. Turn off the heat, and leave the lid on for five more minutes (if your stove is electric, remove the pot from the burner).
After 5 minutes, lift the lid and gently fluff the rice using chopsticks or a rice paddle. Transfer half the rice to the largest bowl you have, or to a large cutting board.
Enlist an assistant if you can: you need to sprinkle the seasoned vinegar onto the rice and toss the rice to coat it evenly, at the same time as you fan the rice to cool it (a small rectangle of cardboard makes an effective fan). Use a rice paddle in a cutting motion so you don’t smash the rice as you mix in the seasoned vinegar. When the rice is evenly seasoned, shiny, and dry, it's ready to use.
Repeat step 6 with the other half of the rice, seasoning with umesu (plum vinegar) and shiso powder. (Alternately, just mix it with furikake or sakura denbu.)