Get about 12" away from the image, centered on the vertical dividing line. Slowly cross your eyes while keeping the left and right panes level ... at a certain point your focus will lock onto the "middle" pane and it will look like 3D. Takes practice to get it consistently.
A "middle, third image" will appear with your eyes crossed. It will be slightly smaller than the two originals. The middle image will appear to float on a plane in front of the two originals. It will be sharp and amazingly "three dimensional" in appearance. The originals will be "out of focus."
A lot of people get headaches when they first try this technique. The "trick" is to let your eyes relax in the crossed position.
These are very small images. I learned the technique reading 3-D x ray films. An x ray image is as large as the part of the body being examined. Generating such a large "3-D" image is very impressive and immersive.