Collect your vegetable scraps while cooking. If you don't have 4 cups of scraps right away, put the scraps you have in a freezer bag. Seal it and place it in your freezer. The next time you're cooking, add new scraps to the freezer bag. Repeat until you have about 4 cups.
Add the frozen vegetable scraps to a stockpot. Optionally, sauté them in a little oil for several minutes. Cover with water. Add in the bay leaf, salt and other spices if using.
Cover with a lid and simmer for 20 to 30 minutes.
Strain the broth through a fine-mesh sieve. Transfer it to air-tight freezer-safe containers, zip bag or large silicone ice-cube trays (read tips in post above).