A good start is to soak a 6 piece of kombu and a small handful
 of shiitake mushrooms in 3 qt of water overnight. Then simmer for
 as long as you can (I try to go for at least 4 hours).
 You can use this as is for a soup stock. Cut up whatever veggies
 you like and add them. If you like a bit more depth, add some miso.