If you're using a chicken carcass, using poultry sheers of a large cleaver, cut the carcass into a few pieces.Add to a large pan and just cover with water. Bring this to a boil and let it simmer for 2-3 minutes - lots of scum will float to the surface. Drain and discard the water and then wash the chicken. Return to the pan and add all the other stock ingredients. Top up the pan with 3.2 qt / 3 litres of cold water.
Heat until the water is just showing signs of boiling. Reduce the heat to low and partially cover. Let the stock cook (barely simmering) for 3-4 hours - try not to stir it too much as you may make it cloudy.
Removing the fat. (there are options!)
Option 1: If you have one, use a gravy separator jug (link in tips below).
Option 2: Let the stock cool, then place in the fridge overnight - the fat will solidify on top and you can easier scoop it with a spoon.
Option 3: Let the fat settle on top, and using a paper towel, rest the towel on top. Most of the fat will be absorbed. Do this a couple of times.
Once you've skimmed off the fat, decant to separate 1 litre/30oz containers and either refrigerate or freeze until you need.
If you can, get hold of a gravy separator - they're great for skimming fat on any sauce or stock. You can buy them here.