In a medium-sized pot, heat the broth until simmering. If you're including mushrooms or raw shrimp (or any other meat or veggies that need extra time to cook), add them now. Simmer for about 3 or 4 minutes, until the mushrooms soften a bit.
While that's going on, slice the green onions into 1/8" slivers. Set aside some of the green tips for garnish, and then add the rest into the pot.
Add the soy sauce, sesame oil, and any tofu or spinach, and let simmer for another minute or so.
In a bowl, scramble the eggs, and mix in the water if you want the eggs to be a little lighter in the finished soup.
Turn off the heat, and then while slowly stirring in one direction (I prefer clockwise, but really, it's your call), drizzle the scrambled eggs into the soup. It helps to hold a fork or chopstick across the top of the bowl, with pointy end hanging over the edge, so you can control the speed as the eggs slide past the tines of the fork (or tip of the chopstick). The eggs will cook immediately, forming lovely strands in the pot.
Serve immediately, garnished with bits of green onion.