You may as well get every cent of value and every molecule of nutrition and flavor possible out of those organic vegetables that you either paid a fortune for or sweated hours in the garden over. Keep a big food-grade plastic bag in your freezer, and throw in all the onion ends, celery tops and bottoms, garlic peelings, tops and cores of bell peppers, mushroom stems, and twigs of herbs that you've dried. If you are so inclined, you can chop these bits into small pieces before freezing them, which makes your stock cook up faster and more flavorfully--but in the midst of dinner or salad preparation I must admit I usually don't take the extra time, but just toss them into the freezer.
I don't use brassicas like broccoli or cabbage leaves in my stock, as I don't like the taste. I like brassicas, just not in my stock, to me they impart a bitter taste but I know other cooks who use them and like the flavor. I don't eat meat, but occasionally eat fish and one summer after fishing I did make a batch of stock with the fish bones etc. in it as well, and it was really delicious; I used it later for making a fish chowder.
When the bag gets full enough, dump the contents into a large stock pot. Just cover the foodstuffs with water, add a bit of sea salt if you are so inclined (I do this, as I sort of feel it helps break the food down a little better and adds flavor, but I don't eat any processed or canned foods and don't really worry about excess salt in my diet; by all means leave it out if you prefer).
Simmer--don't boil--until there's nary an ounce of life left in the veggie scraps and they are all a brownish, colorless mass. Add water to bring the level up to just covering the vegetables if it starts to evaporate and boil off, but don't add too much or you'll wind up with a watery broth. Better to have a nice rich broth that you can water down later if need be.
Pour the hot broth through a colander into another big pot or bowl. Remember to put the pot or bowl under the colander. Yes, I say this through experience--my brain is so used to dumping pasta or things of that ilk into the colander where you save the stuff not the broth that I once poured the stock right down the sink. Sigh.
Send the veggie bits to the compost pile. Strain the stock once more if you like, through a fine mesh seive or cheesecloth. Stir well and pour it into quart or half gallon freezer containers and pop in the freezer to await your fall and winter soup and stew making. Freeze up a couple smaller batches for sauces and gravies if you like. While I usually do this on cool autumn nights, my freezer was jammed full and I needed to make room for incoming vegetables, so I pulled the bag of stock veggies and took advantage of an unseasonably cool July evening to cook up this batch.
Happy thrifty cooking!