A Stereogram is an image that can be perceived with depth perception via one of several viewing techniques. This is the basis of 3D imaging and is in essence assisting one eye to see one image and the other to see a slightly different image. The above gif is a wiggle Stereogram which can simulate the effect but is done with a different technique see here
Naked-eye Stereogram viewing can be a lot of fun and is made much easier to learn with this viewing assistant. After a bit of practice you might be able to view the 3D image without the viewer at all by using the same cross-eyed to uncross-eyed technique explained in this tutorial.
This is a very easy 2 part tutorial to create a stereogram
Part 1 Take images for stereogram.
Part 2 Make Stereogram Viewer.
For Part 1 you will need:
1. app for iPhone: Pic Stitch
For Part 2 you will need:
1. corrugated cardboard
3. craft knife or X acto knife