Step 1: Divide and conquer! Put the canvas in the trash can, cover with seawater, and leave to soak. Set a group to digging a big hole in the sand near the high-water mark, 2 feet deep by 4 feet wide. Send another crew out to collect enough seaweed to make three heavy layers over the fire. Dried seaweed can be added to the trash can to revive it.
Step 2: Line the bottom and sides of the hole with beach stones the size of footballs.
Step 3: Light a big hardwood fire in the pit and let it burn down.
Step 4: Place more stones on the fire, making sure you don’t smother it. Build another fire on top of the second layer of rocks and let it burn down.
Step 5: Place the food in wire baskets or tie it up in loose cheesecloth slings. Dampen the cheesecloth with seawater so it doesn’t catch on fire. You’re now ready to cook.
Step 6: Working quickly, rake the embers away from the rocks, then throw a full 6 inches of wet seaweed over the rocks.
Step 7: Put the potatoes, onions, and sausage on the seaweed and cover them with more wet seaweed. Add the lobster, corn, and clams. Cover with a final layer of wet seaweed.
Step 8: Cover the mound with the wet canvas.
Step 9: Seal the edges with sand and more seaweed so steam can’t escape. Cook for 1½ hours, occasionally dousing the canvas with water. In the last 30 minutes or so, set the butter in a covered pan near the fire to melt.
Step 10: When it’s time to eat, dig the sand and seaweed away from the pit.
Step 11: Carefully lift the corners of the canvas and peel it back, making sure sand doesn’t get on the food.
Step 12: Rake away the layers of seaweed, lifting out the food as it’s exposed. Serve immediately with melted butter.