Place dashi stock in a sauce pan or stock pot, making sure there's enough room for all the liquid plus the extra stuff you plan to put in the soup, set heat to high to bring up to a boil
When the dashi is boiling, lower heat and keep at a simmer
Drain the tofu and pad away excess moisture from the block
Cut the tofu into 3/4 inch to 1 inch cubes
Cut scallions into whatever sized slices you like
Cut shiitake mushrooms in half (or smaller pieces if you'd like)
Throw everything in the pot with the dashi, raise the heat a bit and bring everything up to a boil. Stir until the miso paste has dissolved into the dashi