The typical ingredients in miso soup are tofu, dashi, miso paste and scallions. The dashi is a combination of water, dried black kelp and dried bonito flakes. Also, miso paste comes in both red and white colors.
To make the dashi for the miso soup, mix together 2 cups water and a 2-inch piece of black kelp in a pan over medium heat. Remove the kelp right when the water starts to boil. Then, add in 1/2 cup dried bonito flakes. Simmer the water and bonito flakes for about one minute, then remove from the heat, and let sit for five minutes. Strain out the bonito flakes, then add water, if necessary, to get 2 cups of liquid.
For the miso soup, place the dashi liquid back in the saucepan and bring it back to a simmer. Add 1/2 cup of the simmering broth to 2 tablespoons of red or white miso paste, and stir until the miso is dissolved. Pour the miso and water mixture back into the pan. Add in 4 ounces of tofu cut into small cubes and reduce the heat. Simmer the tofu and broth mixture for one to two minutes, until the tofu is warm.
Just before serving the soup, sprinkle with thinly sliced scallions.