Clean the carcass out of the shell. Use a knife to remove as much of the carcass as possible. Wear gloves and carefully cut around the inside edges of the shell to cut away the flesh.
Put the shell outside in your yard or in a garden. Set it in a place with a lot of insects. The insects will eat the flesh and remains out of the shell. If you do not mind waiting, leave the shell out during the summertime for three months. This will allow the shell to dry and will allow insects to eat out a majority of the remains of the carcass. Otherwise, leave the shell outside for at least a week.
Put on a sturdy pair of plastic or rubber gloves and remove remaining bones and any other flesh still inside the shell. Remove as much as possible with your hands.
Scrub the inside and outside of the shell with an old toothbrush and warm water to remove extra flesh still on the shell. Make sure to scrub the entire shell meticulously.
Turn on a table fan and place the shell in front of it until it is completely dry. After it is dry, it is ready to display.