Usually about a tablespoon of miso to a cup of water/dashi mixture.
First, do the hon dashi - mix with water according to pkg directions, and bring to a boil, let it boil for a minute or so. Turn the heat down, and add the miso - the best way to do this is to put the miso in a bowl,, and ladle a spoonful or so of the dashi broth on top, and mix it well to form a soft, thick 'liquid'. It's easier for the miso to incorporate into the broth that way, and you don't end up with lumps. Once you add the miso back to the pot of broth, don't boil it; simmer gently for a few minutes. Add the wakame (never use Kombu - kombu is a thick, tough seaweed used more for making broths and sushi suu, not eating by itself) and simmer til it's soft. Add the tofu cubes and scallions at the end.
You can also add vegetables, like carrots, spinach, potatoes, and little bits of cooked chicken or seafood, like shrimp.