Photographer Sam Haskins, well known for doing in-camera montage, briefly describes how composite photos were made in the time before Photoshop.
Its a single exposure with the model viewed through optical glass at 45° and the fabric positioned to the side. At the time there was zero retouching after the event. Now of course I have the luxury of scanning the transparency to clean and refine the image in Photoshop - God bless its digital socks.