The easiest way to bring the two images together is to hold up a finger between your face and the monitor. Look at the finger, then shift your attention to the images on the monitor. When they are properly merged, you will see three images. The center image is stereoscopic. Ignore the outer images.
If you have difficulty crossing your eyes, try moving back away from the monitor. This reduces the degree of eye-crossing required.
Once you have the stereoscopic image in focus, try using your hands to frame the center image and block out the outer images. When you get your hands in the correct position, there is an illusion that the 3D image is floating in space between your hands.