Bring water to a slow simmer and add the seaweed. Allow to simmer for at least 5-6 minutes. The longer you simmer the seaweed, the less of a salty, fishy flavor it will have.
Reduce the heat to very low and add the rest of the ingredients. Stir until the miso is well dissolved. It's best not to boil the miso, as this will ruin some of its healthy properties as well as change the flavor of the soup.
Recipe from http://vegetarian.about.com/od/soupssalads/r/MisoSoup.htm, retrieved Dec. 2010.