1) When you have leftover vegetables in small quantities, put them into one freezer container. This is also a good way to save extra vegetables if you won't use them otherwise before they go bad; just wash and cut them in chunks, then put them in the container. The types of vegetables will affect the flavor, but you can use pretty much anything that you would put into soup or stew: carrots, potatoes, peas, green beans, onions, celery, etc. When the container is full, that's enough to make stock -- about 2 cups or so.
2) Save poultry giblets in another freezer container until you have about 2 cups. Necks are best. You can cut off wing tips and tails for this, too. Hearts and gizzards are good. Livers are okay if you love liver, but they crumble and get into everything, so you might not want to use them for stock.
3) When you serve poultry, cut the legs and wings off, then slice the breast meat off. You'll have most of the carcass left with a few shreds of meat, and that's plenty for making stock. Leftover meat can also be used.
4) Other kinds of bones also work, such as a ham or lamb shank, neck bones, knuckle bones, etc. You can buy cheap stew meat and cut the bones and gristle out of it, eat the soft meat, and use the hard bits for making stock. You can mix bones from same or similar animals, like ham and pork neck bones, or lamb shank and goat stew meat gleanings, or turkey and chicken giblets; but don't mix totally different types like ham and chicken bones.
5) Put some vegetables and bones into a crock pot. Spice mildly -- I usually add 1 bay leaf, a few peppercorns, and a pinch or two of salt -- because you'll be using the stock for different recipes later that will have their own seasonings. Fill the crock with water to about 1 inch below the rim. Add a couple teaspoons of vinegar to help leach the calcium out of the bones into the liquid. Turn on LOW and ignore for 4-8 hours. Stir occasionally if you wish. If it looks like it's losing too much water, add a little more.
6) Ladle the liquid into containers for freezing. I usually use containers that are around the 2-cup size. Remember to LABEL them with the contents and date! Put the containers in the refrigerator first; that way you can skim the fat off the top if there is more of it than you want. (EDIT: Stock tends to turn gelatinous when cold. If yours gels like that, don't throw it out!) Then put the containers in the freezer. Discard the solids left in the crock pot.
7) Stock is a wonderful multipurpose cooking base. You can use it as the liquid in a crock pot for making a roast or a bird, or in the oven for basting same. You can thicken it into gravy. You can add fresh vegetables and meat for making soup or stew. You can cook noodles or dumplings in the stock. You can even heat it up to eat plain if you're sick and need something nutritious but easy to digest.