1 Break up larger pieces of shell: Break thick shells (lobster or crab) into smaller pieces by putting in a sealed, thick plastic bag and either rolling with a rolling pin or hitting with a meat hammer to crush.
Cut up thinner shrimp shells with a chef's knife. Don't crush or cut too small. You can even skip this step if you want, if you are already dealing with broken up shell pieces (like cracked crab).
2 Roast shells (optional): Place in a large roasting pan and roast at 400°F for 10 minutes (this step you can skip, but it greatly enhances the flavor).
3 Cover shells with water and heat to not quite a simmer: Put the shells in a large stock pot and add enough water to cover the shells with an inch of water. Heat the water on high. As soon as you see that little bubbles are starting to come up to the surface, reduce the heat to medium.
Do not let the water boil! You want to maintain the temperature at just at the edge of a simmer (around 180°F), where the bubbles just occasionally come up to the surface.
Do not stir the shells! Stirring will muddy up the stock.
Skim the foam. As the bubbles come up to the surface a film of foam will develop on the surface. Use a large metal spoon to skim away this foam. Let the shells cook like this for about an hour; skim the foam every few minutes. The foam comes from shells releasing impurities as their temperature increases.
4 Add the wine, onions, carrots, celery, tomato paste, herbs, peppercorns: Once the stock has stopped releasing foam, add the wine, onions, carrots, celery, tomato paste, thyme, parsley, bay leaf, and peppercorns.
Bring to a low simmer and reduce heat so that the stock continues to barely simmer, but not boil, for 30 minutes. If more foam comes to the surface, skim it off. Add salt and remove from heat.
5 Strain through a lined sieve: Use tongs, a large slotted spoon, or a spider strainer to lift out and remove most of the solids from the stock. (Later put in a plastic bag and put outside in the trash! Shellfish shells have a way of stinking up a kitchen.)
Dampen a few layers of cheesecloth and place over a large, fine mesh strainer, over a large pot or bowl.
Pour the stock into the strainer. Either use the stock right away, or cool for future use.
If you aren't going to use in a couple of days, freeze (remember to leave some headroom at the top of your freezer container for the liquid to expand as it freezes.)