Hollow out the body cap with a knife. Make sure the inside is smooth.
Cut a hole in the plunger where the stick is.
Put the lens in the plunger to make sure it fits. Trim the plunger as needed.
Cut a circle out of the plastic or cardboard that's the same size as the back end of the plunger to make the backing.
Cut a hole in the center of the circle big enough for the lens.
Paint the back of the backing black if you made your backing out of cardboard.
Attach the lens to the plunger using the hot glue or nuts and bolts.
With the black side of the backing in, attach the body cap to the backing.
Attach the backing to the bellows.
Attach the lens to your camera.
Take sample shots from different angles and distances from your subject or object.
Focus your camera using the manual focus on the lens, rather than the digital focus on the camera itself.
Change the aperature by switching to a different manual aperture lens. The camera aperature settings won't generally be good enough for this type of photography.