Put 4 cups water in a large saucepan. To remove any excess saltiness from the kombu, either quickly wash it under cold running water and then pat dry or wipe with a damp cloth. Add to the cooking water, leaving it to soak for 30 minutes
Put the saucepan over high heat and remove the kombu just before the water comes to a boil. Add the katsuobushi, bring back to a boil, and then immediately turn off the heat
Let it stand until all the flakes have sunk to the bottom of the saucepan
Pour the mixture through a strainer lined with paper towel or cheese cloth, leave to cool, and keep the dashi stock in the refrigerator until ready to use. Do not discard the used katsuobushi if you want to make a niban dashi stock. The stock can keep up to 1 week in the refrigerator or freeze up to 1 month