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
Step 1: Part 1. Taking the Images
Using the camera on your iPhone take a photo then quickly but carefully move the camera 6 inches to the right and take a second photo.
Step 2: Part 1. Downloading and Using Pic Stich
Choose the layout in the very center, two portrait orientated, equal-sized rectangles.
Double tap the left rectangle and locate & select the 1st image you took for the Stereogram.
Double tap the right rectangle and locate & select the 2nd image you took for the Stereogram.
Export to photo albums.
Open Photos app and open the image you just saved.