Basically you need to unfocus your eyes and look through the stereogram. Picture becomes blurry and doubled and that exactly what we want. This will make pattern tiles to overlap each other and each eye will be seeing slightly different image. Disparency in views will make your brain see hidden 3D image.
Some people who have problems with binolular vision most likely will not be able to see hidden image. Other vision issues such as nearsightedness and farsightedness will not affect much your ability to see stereograms. Check vision3d.com website for details.
There are different methods to learn seeing stereograms, one or another may work out better for you, though all will require some patience. You can try both stereogram on monitor and printed on paper.
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Please enjoy the three-dimensional image with depth.