This is a combination of recipes I found on the internet. The prep time is an estimate (it will vary depending on how easy it is to remove the lobster meat and whether you decide to pursue the low-yield areas like the legs).
Cut off the fronts of the heads (about 1/2" back from the eyes). Discard.
With back of a heavy knife, crush lobster shells and heads (or coarsely chop) -- or wrap lobster shells in an old kitchen towel and crush into walnut-sized pieces with a rolling pin.
In an 8-quart heavy kettle heat oil over moderate heat until hot but not smoking and brown lobster shells and heads, mashing with a wooden spoon to help break up, about 15 minutes.
Stir in carrot, onion, tomato, parsley, celery, bay leaf, peppercorns and wine and simmer until most of wine is evaporated.
Add water and simmer until liquid is reduced to about 4 cups (about 90 minutes).
Pour stock through a fine sieve into a heat proof bowl. Rinse the sieve and line it with a double layer of cheesecloth. Pour stock onto the cheesecloth to strain it a second time. You can strain it 3 or 4 times if you are a perfectionist. (One blogger who posted a picture of her perfectly clear lobster stock said she strained it 8 times.).
Let cool before refrigerating.
Stock may be made 3 days ahead and cooled, uncovered, before being chilled, covered. Stock keeps, frozen, 3 months.