Hold your thumb in front of your face, about four inches (10 cm) from the tip of your nose. Look at your thumb, but actually direct your attention over it toward the stereo picture on the monitor.
It will probably be out of focus, but the two halves will be nearly overlapped. Adjust the distance of your thumb nearer or farther until you get the stereo picture to lock into one central image.
All you have to do now is focus on the monitor without uncrossing your eyes. This is easy for some people, difficult for others. Just remember that eye muscles are like other muscles; you have to train them and, perhaps, strengthen them. Practicing the above exercize will do this. Think of it as aerobics for eyeballs.
Even if your eye muscles get tired, you will not harm your eyes viewing stereo pictures. Just don't overdo it until you become comfortable with it. If you build up your muscles a little at a time, holding your eyes in the crossed position will soon become as restful as normal viewing.
If you would like more help, look at Greg Erker's viewing pages. He has links to other help sites as well.
To try out your new skill, check out my stereo photographs.