Use a pair of scissors to cut some slits into the sheet of kombu. The kombu might have specks of what looks like powdered salt. That’s umami! There’s no need to wipe it off. The only reason you need to wipe it is if it looks dirty with literal dirt - which shouldn’t happen in this day and age.
Place the kombu in a medium saucepan with 4 cups of water. Turn the heat to medium low. The water should slowly heat up and almost come to a boil. If needed, skim off any foam that rises to the surface. Heating up the pot of water will take about 8-10 minutes, depending on the strength of your stove. When the water starts forming bubbles and almost starts to boil, remove the kombu and set aside.
Add all of the katsuobushi to the pan and bring to a boil over medium high heat. Once the dashi comes to a boil, reduce the heat and simmer for 30 seconds, then turn off the heat and remove from the stove.
Let the katsuobushi infuse the dashi for 10 minutes. It should gently sink to the bottom.
When the 10 minutes are up, strain the dashi through a fine mesh sieve into a bowl or liquid measuring cup. Your dashi is ready to use in recipes or to enjoy as is!
Notes
If needed, you can store the dashi in a tightly covered container for 3-5 days in the fridge.