If you are having trouble with this, wikipedia has some good instructions which helped me. First try autostereograms [1] (these are more simpler to see), and then simple random dot stereograms (RDS) [2], and finally magic eye static and moving stereograms.
BTW, the book which introduced RDS: "Foundations of Cyclopean Perception" by Bela Julesz is considered one of the most influential books in cognitive science. With just a computer, using no invasive microelectrodes and animal sacrifices, he proved a fantastic result...stereopsis (fusion of the individual images from each eye) occurs before object recognition!