Soak the kombu in the water in a large saucepan for 20 minutes.
Put the saucepan on the stove on medium heat and let it come to a simmer, simmering it for 5 minutes.
Turn the heat off, scatter the katsuobushi all over and let sit for 10 minutes at which point the flakes may or may not have sunk to the bottom. Don't fret if yours haven't!
Double line a sieve with the muslin and strain the stock into a bowl. Keep the kombu and katsuobushi for the Niban Dashi.
Niban Dashi
Simmer everything for 15 minutes, strain.
Video
Notes
Total time is the hands on time, it doesn't include any soaking time.