Rehydrate kombu in water for at least 30 minutes or overnight
Turn heat on and simmer kombu for additional 10 minutes. Make sure the water does not start to boil.
Turn off heat and add katsuobushi
Let the katusobushi steep for 15 minutes and remove both kombu and katusobushi from water. Use a strainer or cheese cloth to remove all katsuobushi pieces.
Dashi is ready to use