Put kombu in cold water(let it rest for 20-30 mins in cold water is good, but optional.)

Let it simmer in low heat for 10-15 mins. Make sure kombu is NOT dissolving.

Take out kombu and let the liquid come to the boil.

Add bonito flakes(katsuobushi), turn off the heat after few seconds of rolling boil.

Let it settle down for a minute.

Strain.