Complex sentence: To "advance" the film, the user pulled a paper tab which protruded from the side of the film pack and which was attached, facing the opposite direction, to the junction of each sheet and its intervening section of tape.
Split into simpler sentences: To "advance" the film, the user pulled a paper tab that protruded from the side of the film pack. The tab was attached--facing the opposite direction--to the junction of each sheet and its intervening section of tape.