How do they do an abdominal x-ray?

The source of the: X-ray is a machine where electrons are accelerated in a vacuum to hit a tungsten target, the radiation passes through to a plate, leaving a picture of contrast absorbed (white) vs passed through (black) regions. Water density is grey.

There are: various ways to perform an abdominal x-ray: supine, prone, on your side, and upright. X-ray photons are produced from a tube, directed toward the body part in question, pass through the body, and are detected by film or a digital detector to produce an image.