I saw these when I was a kid, and it took me hours before I finally got it.
I think anyone who has two eyes is capable of doing it, but it is just a matter of understanding and following the proper instruction. If you are having trouble, then follow my instruction. Don't follow anyone else's instruction. They will only confuse you. You need only to listen to me.
When you hold up your finger in front of your face and cross your eyes, then you can split the image of the one finger into two fingers. In the case of these stereograms, it is not a matter of
splitting the images by crossing your eyes--it is a matter of
merging the images by crossing your eyes.
This image is probably the easiest to start on. There are a series of dark vertical seams that you can easily see normally. Pick a vertical seam in the middle, and cross your eyes so that one vertical seam is merged into an adjacent vertical seam. Once you achieve that overlap, then the 3-dimensional image becomes visible.