Soak the kelp in a large bowl overnight (about 10 hours) with 3 quarts of water. Remove the kelp, reserving it for making a second round of dashi. This soaking liquid is the kombu dashi (kelp stock) and is used when a recipe calls for kelp stock.

Pour the kelp stock into a cooking pot and set it over medium-high heat. When it comes to a boil, add the fish flakes all at once. Count 10 seconds, then quickly turn off the heat. Remove any foam that rises. Let the stock rest with the fish flakes for 2 minutes, then strain it through a fukin, tightly woven cooton cloth, or a strainer lined with a strong paper towel. Discard the bonito flakes or reserve them along with the reserved kelp for making a second round of dashi.

If you forgot to soak the kelp overnight, put the kelp and measured cold water in a pot, set it over mediumlow heat, and cook for 10 minutes. Remove the kelp and turn the heat to medium. When the kelp stock comes to a boil, add the bonito fish flakes and follow the preceding instructions.

To prepare a second round of dashi, combine 3 quarts of cold water and the reserved kelp and bonito fish flakes in a pot, set it over medium-low heat, and cook for 10 minutes. Strain the dashi through a fukin or a strainer lined with a strong paper towel, discarding the bonito flakes and kelp. Use the second round of stock to prepare a miso soup whenever you like. Refrigerate or freeze the unused second round of dashi in sealable containers in small batches.