Basically, you sandwich the bones you cut out in between two pieces of white paper and a frame on either side. She suggests that you can use an old cereal box to cut out the frame. Then, hold a flashlight or light behind it to see the X-ray and make the diagnosis.
A great way to make a doctor kit even more exciting- especially for the slightly older sect. Visit A Bit of This and a Bit of That for the complete tutorial and more photos.