Complex sentence: Alternatively, a piece of paper with a small opening cut into it can be used in a similar manner; when correctly positioned between the image pair and the viewer's eyes, it will seem to frame a small three-dimensional image.
Split into simpler sentences: Alternatively, a piece of paper with an opening of suitable size cut into it can be used in a similar manner. Correctly positioned between the images and the viewer, it will appear to be a frame around a small three-dimensional image.