In a medium-sized pot, bring the sake to a low simmer over medium-high heat for about a minute or two.
Stir in the mirin and the soy sauce.
Add the kombu and the katsuobushi and stir again. Make sure all of the katsuobushi flakes have been immersed into the liquid. Bring it to a simmer and let it cook for about 5 minutes.
Let it all simmer for about 5 minutes, then remove the pot from the stove; set aside to cool. Once it's cooled a bit, strain the mixture through a fine mesh sieve or strainer, pressing on the katsuobushi with the back side of a spoon or spatula to push out as much of the liquid as you can. Every drop counts!