Freeze crab shells, shrimp shells, fish heads, etc. until you have enough to fill a stock pot. Then, place it all in a stockpot, cover with water and boil for about 30 minutes. Remove all the solid pieces. Continue to boil the remaining stock until it’s reduced by about one half. This should leave you with about 2 1/2 quarts of stock. I don’t add any additional seasoning because the shrimp and crabs have a lot of salt naturally. Season to suit your taste. Package in quart size freezer containers or bags.