Cut off the bottom off the abdomen below the second pair of book lungs. (Otherwise, it has a tendency to fold in on itself.)
Prepare a dish of water with a drop of dish soap to reduce surface tension. (Otherwise, the light molt has a tendency to float on top.)
Place the molt in the dish. Let it soak for up to five minutes (until it becomes pliable).
Photograph the inside of the abdomen, focusing on the area between the first pair of book lungs. Sometimes it helps to backlight the molt.
Lately, instead of removing the molt from the water before taking photos, I've been spreading it out in the water and taking the photo while it's in the water. It's a lot easier to spread the molt in water (it tends to fold on itself when you remove it), and the water acts as a diffuser of light.