This is the recipe that Lidia and I use a lot at home, especially during colder weather, and when we don't feel like being creative.
(I'm not used to giving recipe notation and directions, so please forgive any gaps or lapses...)
Combine broth and water in saucepan, bring to boil, then reduce heat and let simmer. Place miso in a heat-proof cup, add about one cup of the heated liquid and stir vigorously until miso is mostly dissolved.
Pour miso mixture back into saucepan (do not let boil) and stir briefly. The soup is now basically done, and this is where you can add the optional ingredients. As you can guess, my additions make it more of a sweet-and-sour dish, and the soy sauce is for Lidia, who likes her Asian food salty.
After you've made sure everything is dissolved that should be, you ladle this mix over the chopped tofu that you've placed in the soup bowls, and sprinkle the whole thing with the chopped onions. In the past, we've added a dash of toasted sesame seed oil at the last minute.
Serve with a cup of steamed brown or white rice, and that's a decent dinner for four. You start the rice, and make the soup meanwhile, and it can take you as little as 20 minutes (more if you use brown rice, which I _love_ with this soup!)
I have seen other receipes for miso soup that are considerably more complicated; we tried one in the Moosewood Cookbook that used a lot of veggies and time, but it was good, too. Hope this helps!