The left picture of the pairs is viewed by the left eye and the right picture of the pair is viewed by the right eye. With practice you can relax your eyes and so that you will seem to see three pictures with the centre one having the stereo effect. Some people find it helpful to block the pictures not viewed by either eye by placing a narrow card or fingers between the eyes and the pictures. A position can be found for the fingers so that the right eye only sees the right picture but is blocked from seeing the left picture. Similarly the left eye will only see the left picture. The positioning depends on the eye separation and the size of the pictures as shown in this diagram:
When the fingers are correctly positioned, closing the right eye allows the left eye to see only the left picture, then closing the left eye allows the right eye to see only the right picture. With both eyes open the two pictures merge into one stereoscopic image.
Special glasses can be bought to help viewing these pairs. Such glasses can be made from cheap 3 dioptre reading glasses as shown here.