Place the Kombu and the water into a jar with a lid or leave it in the pot you will be using with a lid on top. Let it sit at room temperature for 30 minutes or sit overnight on the counter or in the fridge. It should not be left in a hot place. 

You want to slowly heat up the stock. When it is about to come to a boil, quickly remove the kombu from the pot. If the kombu is overcooked it becomes slimy and will result in a slimy and bitter stock. 

Once the kombu has been removed the katsuobushi can be added. Once the stock has come to a boil remove it from the heat and let the katsuobushi sit for five minutes.

Strain the broth. Press out all the stock with a wooden spoon and a fine mesh sieve or use a cheesecloth. 

You now have Ichiban dashi. You can store it in the fridge, freeze it for later or make miso soup.