Making your own vegetable stock is pretty easy, it really doesn't take too long, and makes a hugh difference in the quality of your soup. If you're planning ahead or if you hate to throw anything away until its fully used (even before it hits the compost!), hang on to your clean vegetable scraps (tops, peels, stems, seeds, ugly or tough parts) whenever you're cooking. You can freeze the scraps until you're ready to make stock. If you're about to make soup, prep all your veggies first, save your scraps and use them to make your stock.
This method works best if your making soup with lots of scraps (corn cobs or squash skins & seeds) or if you've squirreled away veggie scraps in your freezer. If your scraps are meager, add an onion, a few carrots, a few celery ribs and a few crushed cloves of garlic for a very basic stock. Rough chop veggies before adding them to the pot.
I start with a little olive oil in the pot (on medium-high heat, stirring occasionally) and caramelize the veggie scraps before adding water for a rich vegetable stock. Once you've got a nice fond (but do not burn!) in the pot, add your herbs (such as a bay leaf, a few springs of fresh parsley and thyme, a few fresh sage leafs) and give a stir while it cooks a couple minutes. Then add enough water to cover your veggies (if you've saved bean broth from cooking beans from scratch or a parmesan cheese rind, you can add that now, too), a bit of sea salt and bring to a boil. Cover and simmer for 30 minutes. Strain out the veggies and your stock can go straight into the soup (or freeze it for later).
What if the stock is weak?
The easiest fix is to boil the stock down to concentrate the flavors. Or you can add "cheater" ingredients to richen the flavor: veggie bouillon cubes or powder (this is the cheater ingredient I keep on hand), sprinkle in a bit of nutritional yeast, or liquid amino acids.
What doesn't go in the stock?
Never use rotting vegetables; send those straight to the compost bin (the quality of your food can only be as good as the ingredients that go into it). It is also a good idea to avoid using veggies that get stinky when overcooked (like broccoli, cabbage, kale, turnips, etc). I also avoid onion and garlic skins and pepper because I think they make the stock bitter when boiled for a long time. Don't use strong tasting (or strong colored) veggies or herbs (fennel, beets, basil, rosemary...) in the stock unless they are also used in the soup.