Stereo graphic images can be viewed as though looking to the horizon through a window. If the two images are spaced at the right distance, the 3 Dimensional image will come into focus when the eyes are most relaxed.
Step 1: Start with the images
Start off staring at the two images.
Step 2: Relax Eyes
Now completely relax your eyes and go out of focus (focus on the horizon).
Tilt your head until you see three images line up like so.
Step 3: Focus On the Center Image.
Now focus on the details inside the center image.
Tilt your head if needed and view image as though looking through a window.
Relax and the center window will come into 3D .
Step 4: May need to Scale the Images
The 3D viewing is easiest when the width of one picture equals the distance between the eyes. Sometimes a image may need to be scaled in size to do this.