I either bake or cook a whole chicken (or even any chicken parts that are bone-in). When the chicken is done and the bones are picked, I put all of the bones and skin and any juice leftover from cooking in the crockpot.
I add water to cover the bones and turn it on low for 10 hours.
In the morning I strain the stock out and throw everything else away. It will be darker than the store bought stock.
I put it in a bowl, cover it and put in the fridge until the fat separates and rises to the top. Scrape the fat from the top and dispose of it.
Now you have a gelatinous stock. Mix it up in the bowl because seasonings fall to the bottom. I learned the hard way that it is best to measure your stock before you freeze it. I put 1 cup of stock per freezer baggie.
I label them with the amount, what it is, and the date and then I put them on a pan to freeze them flat. This way they don't take up much room in the freezer.
You can get anywhere from 4 to 8 cups of stock depending on how much water you use and how big your crock pot it. If you know in advance that you are going to need it, pull it out the night before to give it time to thaw. I tend to forget to take it out in advance and just cut the baggie off of the frozen stock and put it in a pot to thaw and heat it.