1.Gently clean the kombu with a damp cloth without removing the white powdery "umami" substances. Do not wash the kombu. Make a couple of slits on the kombu. Put the kombu and water in a saucepan for 3 hours or up to half day.
2.Slowly bring to a boil over medium low heat, skimming the surface occasionally. Just before boiling (you will see bubbles around the edges of the pan), remove the kombu. Turn off the heat to let the dashi cool down a bit.
3.Add bonito flakes and bring it to a boil again, skimming occasionally. Once the dashi is boiling, reduce the heat, simmer for just 30 seconds, and turn off the heat. Let the bonito flakes sink to the bottom, about 10 minutes.
4.Strain the dashi through a sieve lined with a paper towel set over a bowl. Gently twist and squeeze the paper towel to release the extra dashi into the bowl. If you are not using the dashi right away, save it in a bottle and keep in the refrigerator for 3-7 days or in the freezer for 3 weeks.