1. Make a couple of slits on the kombu to help release flavor. Put the kombu and water in a medium pot and let sit for about 6 hours.
  2. Turn on the heat to medium-low heat and slowly bring to almost boil, cleaning the dashi by skimming the surface. Just before the dashi starts boiling, remove kombu from the pot. If you leave the kombu in the pot, the dashi will become slimy and bitter.
  3. Add the katsuobushi and bring it back to a true boil for 30 seconds, then turn off the heat. Let the katsuobushi sink to the bottom, about 10 minutes.
  4. Strain the dashi through a fine-mesh sieve. Dashi is ready to use.