Note: This recipe fits a 7-quart (6.6 L) slow cooker. If you have a smaller crockpot, you may need to use less water and fewer veggie trimmings.
Once you've collected sufficient veggie trimmings, put them in your slow cooker with the water and apple cider vinegar. You are also welcome to add additional spices or bones (if desired). I added a few chicken feet to this batch. Make sure you use enough water to fully submerge the vegetables.
Put the lid on your slow cooker, set it to low, and let it cook for 24 hours. You can skim off any scum that rises to the top of the pot during the cooking period.
After cooking, put a large colander over a large bowl. Line the colander with a layer of cheesecloth. Strain the stock through the cloth so that it collects in the bowl. Discard the vegetable trimmings.
Let the stock cool. If it will be used within a couple of days, you can store it in glass canning jars in the refrigerator.
Any stock that will not be used within a couple of days should be frozen. Pour it into cocktail ice cube trays and place the trays in the freezer. After freezing, remove the cubes from the trays and store in freezer bags.