Prepare the Soup Base: Add dried sardines, kombu, and water in a pot and heat until just before boiling. Remove the sardines and kombu.
Add Bonito Flakes: Add thick bonito flakes and boil for a few minutes. Turn off the heat, add thin bonito flakes, and wait for about 5 minutes until they sink.
Season the Soup: Filter the soup with a paper towel. Season with soy sauce and a pinch of salt.