Prepare a bowl of cold water to wet your hands. (This will prevent the rice from sticking to your fingers)
 
For cooking sushi rice :
Dissolve the sugar in the rice vinegar.
In a fine strainer, rinse the rice under running water until the water is clear.
In a saucepan, weigh the rice and add 1.5 times its volume in water. Bring to a boil and cook for 15 minutes over low heat and covered.
Let the rice cool down.
Pour the sugar-vinegar mixture over the rice and mix with a bamboo spatula.
Shape a well-pressed elongated ball of rice in your wet palms.
Place on a bamboo board. Repeat the operation to finish the rice.
Cut thin slices of red tuna and place them on the rice balls.
Refrigerate until serving.