This is a typical stereogram.
First, make sure that you’re squarely in front of the picture, not off to the side at all. Once you’ve got it centered right in front of you, notice that there are 2 dots at the bottom center of the picture. There are two ways to proceed - you can either (1) cross your eyes, or (2) let your eyes unfocus, until those two dots become three dots … not four, but exactly three dots. It may take a bit of practice. Persevere. It’s worth it.
When you can see three dots at the bottom instead of two, you’ll notice that the rest of the stereogram has come into focus.
The picture just below is what’s actually hidden in the picture above, only when you see it above, it will look more like a cone because of the shading. It takes up the entire picture from top to bottom, and all but a little bit on either edge.
And so now, my stereographically challenged friends … you have something to actually practice on! Keep practicing it until you don’t have to concentrate on it to make it happen. Once you can do it comfortably, go back to the others I’ve posted, and take a look at them with new eyes.

Before I got addicted to these, my kids used to bring home books full of them from the library. They would get all excited … “Hey look! It’s a cube!” … or “Moof! It’s a dog! Can’t you see it?” I was certain that they were making it all up … just to annoy me.
One day, I got tired of trying, and I just sat in front of one of the stereograms, with my mind wandering off into other pastures. As I sat there, I slowly realized that I was seeing an amazing three dimensional scene on the page in front of me. I nearly died from the shock! It surprised me so much that I lost the 3D picture. However, now that I knew it was there, I wasn’t going to let the darn thing get away! It took some effort to get it to come into focus again … but each time I did it, it was easier. Now, I just look at stereograms and see them right away.
I found a really nice animated one yesterday when I was looking for these examples … I’ll post it after people have had a chance to work on this “lesson.”
Good luck! :o)
To Be ... Or Not To Be ...:
Sneeze/Nausea Connection - March 2007 Update:
It Must Be a Miracle!: