1. Place the sheet of kombu in a pan large enough to hold it flat and cover with cold water. Soak it for 8 hours or overnight at room temperature.
2. Remove the kombu from the water, and cut it into 8 pieces with a sharp, heavy knife. Pour 2 cups of the soaking water into a 1-quart saucepan, add ¼ cup vinegar and the cut-up kombu and bring to a boil.
3. Reduce the heat to its lowest point cover the pan, and simmer for 1 hour, or until the kombu is tender. Then stir in the sugar and soy sauce, replace the cover, and simmer for another 20 minutes.
4. By now most of the liquid should have cooked away. Let the kombu rest off the heat for 20 minutes before removing the cover.
