Open full view then cock your eyes (like your looking through your monitor) until you see 3 images. Focus on the image in the center. You may have to sit back a few feet from your display depending on size if you cannot get the images to merge.
I really think you mean to tell people to cock their eyes. It's a lot easier to look at large stereograms like this by crossing your eyes, but it doesn't look correct. It looks as if I'm looking at the underside of the image.