Check kombu and wipe clean any dirt or grit (usually not needed for today's commercial preparations.) DO NOT wipe off white substance (mannitol) as it contributes to the umami. Make a couple slits in the kombu to help release more favor.
Put kombu into the water to rehydrate for a minimum of 2 to 3 hours in the summertime or 4 to 5 hours in the winter time. It is best though if left to brew overnight.
Slowly bring kombu dashi to a boil over medium-low heat. Just before the dashi begins to boil, remove the kombu. If left in the kombu becomes slimy and the dashi, bitter.
Stop here if you are vegetarian/vegan.
Awase Dashi:
Allow kombu dashi to come to a boil and add katsubushi. Let simmer for 30 to 60 seconds.
Remove from heat and let steep for 10 minutes. Strain dashi through a fine mesh sieve.
Store dashi in the refrigerator for 3 to 5 days or in the freezer for 2 weeks.