Bring water to a boil in a large pot, and add soy sauce, spinach, mushrooms, scallions, carrot, and celery.

Boil for several minutes, until the vegetables are tender. Reduce heat to low, remove some of the broth (a cup or so) and transfer it to a bowl. To this bowl, add miso and whisk the miso into the water until there are no lumps left.

Return the miso mixture to the pot and blend with the rest of the broth. Cut the silken tofu into cubes (whatever size you want them to be when you eat them), and stir into the soup.

When the mixture is heated through, the miso soup is ready to serve. Make sure you give it a good stir before ladling it into bowls -- the miso tends to settle to the bottom.

Note: A widespread claim about miso is that it has nutritive properties that disappear once the miso is boiled. I am skeptical of this claim, but on the other hand, it doesn't take much effort to make sure the soup doesn't boil once the miso is added in, so what the heck. Use this information as you see fit.