You have to cross your eyes until you see three images. The middle image is the one that will be in 3D. Let your eyes relax until the middle image comes into focus.
I can tell that these pictures were taken at two different times. In the bottom of the one on the left, there is a small brown leaf, but it's not there on the right.
All you do is focus your eyes on something far away and look at the picture while your eyes are still unfocused.