Start with your face close to the screen. The "+" will appear to be inside the "o". (If your screen is large, you may have to back away a bit.) Relax your eyes, and slowly back away, maintaining the effect. (This feels the same as viewing "Magic Eye" or "Random Dot Stereogram" pictures.) Try this with a photo pair. When there appear to be three images, concentrate on the middle one until it becomes 3D.
"Parallel-Viewing" may be more difficult on a large screen if the width of any single image is wider than the distance between your eyes. Reducing image size or sitting farther away may help.
CAUTION! If you experience eye fatigue or discomfort, take a break!