Place chicken carcasses (or beef bones) in a large stock pot, along with (optional) vegetables and salt and pepper, if you choose to season. Using meaty pieces will give you a broth, whereas using just the bones and carcass makes a stock. I usually make the plain stock, without the addition of veggies and seasoning, but most people choose to add these things. Either way works!
Cover with water so they are completely submerged. Simmer on low all day long. (You'll want to start this process in the morning). As stock simmers, occasionally skim fats and foam off the top with a mesh strainer.
Once stock has simmered all day, strain into a large container, through a mesh strainer to remove last of debris, cover and refrigerate. The hardened white layer on the top (the fat) can then be removed. The gelatinous consistency is good! That means you have made a good stock--this is not the fat, this is collagen and other nutrients.
You may can right away, or the next day, whatever your schedule allows. When you are ready to can, heat your canner up with an inch or so of water, as you do to can anything else, and reheat your stock to a boil in a large stock pan, to reliquify. Ladle your stock into pint or quart jars, leaving a half to one inch head space, and seal with lids.
Refer to your pressure canner's manual for the correct setting to process at; it needs to be adjusted by altitude, and process for 20 minutes. Once finished, and jars are cooled, they can be stored in the pantry for future use.