Preheat the oven to 150℃/375℉
Place the chicken carcasses (or a whole chicken) in a roasting dish and roast until the meat is cooked though, about 40 minutes. This will be a different time depending on how much meat is left on the carcass. If you're working with a whole bird - cook the the correct time for the size you're working with.
Once the meat has cooled enough to handle, pull the easy to remove parts off the bones and reserve it. You can add it back into the soup, or use it for a different dish.
Place the bones, skin, and any giblets into a stock pot or dutch oven skillet and add a roughly chopped onion, salt and pepper. You can also add any unwanted veggie scraps you have at this point to cook down and flavour the broth.
Fill the pot with water, enough to fully cover the chicken and anything you've added, about 2 litres.
Bring the pot to a simmer and leave it safely simmering for 2-3 hours. If the water gets low, top it up to just covering the bones.
When it's had a 2-4 hours to simmer, carefully strain and reserve the liquid. You'll have about 1.5 to 2 litres of broth. This is your broth for making the soup or using for anything you want.