Gently wipe the kombu with a kitchen towel. Cut slits into the kombu on both parallel sides.
(Optional: If you have extra time the day of making dashi, soak the kombu in 500ml of water in a pot for 2-6 hours.) If not, start to boil the water with the kombu inside over a medium heat. Right before it boils, remove kombu. Turn off the heat and let the dashi rest for about 10 minutes.
Add in the bonito flakes and bring it to a boil over medium heat once again. Once boiling, turn off the heat and let the dashi sit for about 10 minutes.
Take a container of your liking to store dashi and strain into it. Make sure no bonito flakes are in your dashi.