If you're really lazy, you can buy packets of powdered soup or pre-mixed miso paste and dashi concentrate. However, if you're a bold young chef eager to commit brave acts of culinary derring-do, you can try this recipe:
Le Traditional and Butt-Plain Miso Soup
1 cup secondary bonito stock (from scratch or from dashi powder)
Heat the bonito stock. While you wait for the stock to come to a boil, blend the miso paste with a little stock to make it easier to mix into the soup. Just before the stock boils, add in the miso paste. When it boils, take it off the heat and serve.
...And that's it. Soup for one.
If you're even braver, you can try adding things to the soup. There are a number of different aestheticsmy favorite is the "one ingredient which sinks, one which floats" rulebut the final rule is, "Whatever tastes good."
When adding ingredients, take care to see that everything arrives at the peak of doneness at the same time. You'll need to add ingredients to the broth at different times.
Garnishes and seasonings (optional)