Wipe away any dirt from the kombu with a kitchen towel, being careful not to rub off the white powdery deposits. Place the kombu and water in a saucepan and allow it to soak for 30 minutes to become soft.
Remove the kombu from the water and cut several lengthways slits into the leaf. Return the kombu to the water and bring it to the boil. As soon as the water begins to boil, remove the kombu to prevent the stock from becoming bitter.
Stir the bonito flakes into the kombu-flavoured water, bring back to the boil and take the pan off the heat. Allow the water to cool. When the bonito flakes have settled to the bottom, strain through a sieve lined with cheesecloth or a coffee filter.
Dashi kombu and dried bonito flakes can be purchased in Oriental speciality stores or online.