I prefer to soak my konbu for 24 hours to make my dashi, but there are those times where one forgets to do so and a dashi is needed. This method does take some time, about 1 to 1 ½ hours to make, but produces a great dashi. For this particular dashi I add dried shiitake to increase the umami.
In a medium-sized pot, soak the konbu and shiitake in the water at room temperature for at least 30 minutes, and up to 1 hour (the longer the better).
Turn the burner on low and heat for 30 minutes. This is very important: you want the water hot, but not close to boiling at all.
Add the katsuobushi and strain immediately through a chinois.