Wipe the kombu with a damp cloth to clean it, then cut into 2.5cm pieces, and place into a saucepan along with the water. Let stand 15 to 20 minutes.
Bring the kombu to a rapid simmer; reduce to a slow simmer over low heat. Sprinkle in the bonito shavings and simmer a few minutes more until the flakes have sunk to the bottom of the saucepan. Remove from the heat and let stand a few minutes. Strain through a mesh strainer before using.