If you are still using x-ray film or if you have films from the past and you want to create digital versions of your films you can either scan them with a transparency adaptor or take a digital photo of the film on a light box.
Getting a good diagnostic backlit camera image can be tricky. The linked article from Dennis Braunston has some tips on how to do it.
If you are taking a picture of a single x-ray, like a bitewing, then you will need to attach the cameras special magnifying adapter or with other cameras zoom in until you frame the image . For a panorex or a full FMX then leave the adapter off. The example given is for an FMX.