1) View the two pictures at a normal, comfortable distance.
2) Make sure left and right eyes at the same level.
3) Put the tip of a finger in between the eyes and the pictures about 150mm (6 inches) from your eyes. Focus your eyes on the tip.
4) Slowly move the finger forward and back until there appear to be only 3 pictures.
5) Shift the focus of your eyes to the central picture without un-crossing your eyes - this takes a bit of practice.
6) The 3D aspect of the central picture should suddenly be apparent.
7) If the picture appears blurry, try tilting your head slightly from side to side so your eyes are level with the picture.