Wash the rice in cold running water 4-5 times or until the water runs pretty clear. Drain off all the water
In a medium saucepan with lid, put in the rice along with the water and kombu seaweed if using. Bring the water to a boil. Once it boils, use a tong to get the kombu seaweed out and discard it. Lower the heat, cover with lid and let it simmer gently for the next 10 minutes. Turn off the heat. DO NOT OPEN THE LID. Let the rice sit for 15 minutes
While waiting for the rice, in another small saucepan, put the rice vinegar, sugar and salt in the pan and bring it to boil until the sugar dissolves. Set aside. After letting the rice sitting for 15 minutes, open the lid and transfer the rice to a wide shallow plate to cool it down. Pour the seasoning over the rice and use spatula to gently mix it (don't break the rice grain and turn it into a mush). Use your other hand to fan the sushi to cool it down quickly. If you have a fan, you can do this in front of the fan (even better!)
Now you should have a glistening-looking sushi rice at room temperature when you are ready to use it
Notes
I suggest to prepare the rice the same day you're going to make the sushi. It doesn't take that long at all and you don't even need a rice cooker to cook the rice