Use the Magic Eye book if you have it or go online to magiceye.com for samples of the 3D images. Many people find it hard to stare closely at a computer screen for long, however. The computer screen should work fine for short practice session, though.
Bring the picture right up to your nose. Focus your eyes as if they were looking past the picture at something about a foot away. Ignore the picture in front of you and let your eye muscles relax.
Start to slowly take the picture away from your face. Keep your eyes relaxed and focused on the point past the picture. This is the key for your eyes to see the 3D image.
Practice setting your eyes in 3D by staring at two dots on a page. Bring your eyes close like before and look through them until you see three dots. If you see four dots, slowly bring your eyes away from the page until the dots merge.
Repeat the process until your eyes get comfortable and you can see the image over and over again.
Move your eyes back and forth slightly to bring out the 3D feel of the picture.