Instructions:

If this is the first time for you, try to keep your eyes relaxed. Bring your eyes right up to the screen and slowly pull your head back. Don't try to focus on the image but rather, allow your brain to bring the hidden picture into focus.

There are two methods worth trying if you have difficulty. You can try to go cross-eyed and see how you go, but this can be difficult. The other way is to try to keep the pupils of your eyes from converging on the image. They must strike the image at about 90 degrees. This seems to happen if you bring your head up to the screen and pull back without allowing your eyes to focus. With practice, the image will emerge from your fuzzy field of vision. At that point you will be able to relax more and even move your head around without losing the image.