Step 1Using scissors, make several crosswise cuts in the konbu. This helps to extract the flavor during cooking.
Step 2Place the konbu and 8 cups of water in a large saucepan and set it aside for 10 minutes. Place the pan over medium heat and cook until the water almost comes to a boil, then pluck out the konbu.
Step 3Keep heating the water to a boil, then remove from the heat. Add one-half cup of cold water. Wait for the liquid to cool a minute or two, then gently add the bonito flakes. Do not stir. When the bonito flakes have settled, about 3 minutes, strain them through a sieve lined with cheesecloth or paper towels. Don't stir the stock because that will cloud the dashi. The dashi should be aromatic and have a light amber color.