4 pounds chicken carcasses, including necks and backs
1 large onion, quartered
4 carrots, peeled and cut in 1/2
4 ribs celery, cut in 1/2
1 leek, white part only, cut in 1/2 lengthwise
10 sprigs fresh thyme
10 sprigs fresh parsley with stems
2 bay leaves
8 to 10 peppercorns
2 whole cloves garlic, peeled
2 gallons cold water

Place chicken, vegetables, and herbs and spices in 12-quart stockpot. Set opened steamer basket directly on ingredients in pot and pour over water. Cook on high heat until you begin to see bubbles break through the surface of the liquid. Turn heat down to medium low so that stock maintains low, gentle simmer. Skim the scum from the stock with a spoon or fine mesh strainer every 10 to 15 minutes for the first hour of cooking and twice each hour for the next 2 hours. Add hot water as needed to keep bones and vegetables submerged. Simmer uncovered for 6 to 8 hours.

Strain stock through a fine mesh strainer into another large stockpot or heatproof container discarding the solids. Cool immediately in large cooler of ice or a sink full of ice water to below 40 degrees. Place in refrigerator overnight. Remove solidified fat from surface of liquid and store in container with lid in refrigerator for 2 to 3 days or in freezer for up to 3 months. Prior to use, bring to boil for 2 minutes. Use as a base for soups and sauces.

Source: http://www.foodnetwork.com