As you are cooking with vegetables, collect any peels or stalks that you cut off and place them in a sealed freezer bag in the freezer.
Once you have collected about 4-5 cups of vegetable scraps, pour the frozen scraps into a soup pot and cover it with 8 cups of filtered water and 2 teaspoons of salt.
Turn the heat to medium-high and bring to it boil, then turn the heat to low and simmer uncovered for 1-2 hours. (The longer that you simmer, the more concentrated the flavor will be.)
Place a strainer on top of another pot and slowly pour the broth through the strainer.
Allow the broth to cool a little, then store it in mason jars or super cubes until you are ready to use it in recipes.
Notes
Don't use rotten vegetables that have a bad smell, they will also have a bad taste.
Avoid peelings from non-organic vegetables if you are worried about chemicals or pesticides.
Wash any dirt or soil off the ends of the stems.
Use 1 teaspoon of salt for a low-sodium vegetable broth.