You can use raw or cooked chicken carcasses. I usually roast two birds then make stock the next day and this will give me 3-4 portions of stock. The vegetables give the stock more flavour but if you don't have them its fine to make a stock with just seasoning.
The longer it simmers the more the bones break down releasing good stuff into the stock.
Ingredients (per carcass)
1/2 dozen black peppercorns
1/2 onion, roughly sliced
1 small carrot cut into chunks
1 stick of celery
Put all the ingredients in a large pan and cover with water (ideally filtered)
Bring to the boil then simmer on a low heat for at least 3 hours.
Strain off into glass dishes, allow to cool before storing