Place kombu and water in a large stockpot and let it soak at room temperature for at least 20 minutes. You can soak longer, too, even overnight, which will allow the kombu to release more flavor. Bring to a boil over high heat. Remove the kombu and decrease the heat so the liquid is simmering. Add the katsuobushi and gently mix into the liquid; don’t stir vigorously. Simmer for 10 minutes longer, then strain through a fine-mesh sieve.
