It took me a while to learn how to cross my eyes and focus on the third image in the middle, but once I got it, it became easier every time. I am fascinated by these... some of them blow me away when they suddenly turn into vivid 3D!
These examples are all just my own, and certainly NOT the best ones out there, but I still have fun with them. I hope you'll spend a bit of time looking at them, trying to get the hang of the cross-eyed technique.