Try the following, using a simple, well defined image:
Sit so that your eyes are about 2 to 3 ft back from the screen. Make a frame by putting up both hands palms out vertically about 1 ft in front of you, with thumbs extended, pointing towards each other and overlapping, and forefingers about 2 inches apart. Move this frame to and fro between your eyes and the screen, until you can see only the left picture with your right eye, and only the right picture with your left eye. Now open both eyes, and let your eyes relax. With practice, a single image in 3-D will form in the frame. The focus of the 3-D image appears to be in front of the screen. Further practice will allow you to dispense with the frame, when the 3-D image will appear as a third image between the two real images.