Remove the tops/bottoms/skins/stems from any vegetables you are preparing (avoid vegetables like Brussels sprouts, broccoli, or cauliflower as they will add a bitter taste to your stock) and place them in a ziplock bag - they can stay frozen up to 6 months.
Note: You can add many other vegetable scraps (think sweet!) - i.e. corn cobs, winter squash, zucchini, and other squash, beet greens, fennel, chard, lettuce, parsnips, green beans, pea pods, bell peppers, eggplant, mushrooms, asparagus, and herbs like dill, thyme, parsley, cilantro, and basil.
Continue like this until bag is full.
Dump bag into pot and fill ¾ of the pot (or until scraps just start to float) with water.
Bring water to a boil and then let it simmer for at least 30 minutes.
Strain water out of stock.
Refrigerate stock up to 4 days, or freeze up to 3 months.