Soak the kombu in the water for at least an hour. Preferably overnight.
Put the kombu and the water in a pan and heat until just below boiling. Remove the kombu.
Bring the water to a boil. Just as it starts to boil, take it off the heat and add the katsuoboshi.
Let it cook at a low heat for 4-5 minutes.
Strain out the katsuoboshi using a fine mesh strainer. Line with a paper towel if you need to.