For making delicious soup stock, you should buy high quality dashi-kombu,thick and straight, as much as possible.
Place water in a saucepan. Soak the kombu and shiitake mushrooms in the water for at least 15 minutes, until they become tender enough. (If time permits, more than three hours to overnight is much better.) Heat the water over high heat and reduce heat once it boils. Remove kombu just below boiling point.
After around five minutes, remove saucepan from the heat. The boiling time depends on the size of shiitake mushrooms and the soaking time.
Remove the shiitake mushrooms from the water, and save them for use in other recipes.
Notes: Kombu and dried shiitake mushrooms are available in Oriental stores. You can make dried shiitake mushrooms by drying raw shiitake mushrooms in the sun for a couple of days.
You can make the soup stock with one ingredient, kombu or dried shiitake mushrooms. In this case, double the portion of the chosen ingredient and soak longer. If you cook with only shiitake mushrooms, it's better to soak them in warm water. For making thick dashi, increase the ingredients or soak them longer.