This process involves tricking your eyes into merging the two images on a stereo card into one 3D form.
It can sometimes take a few minutes to "get" the process at first, but becomes second nature after a while.
Contrary to popular concern, cross-eyed free-viewing doesn't actually require you to cross your eyes
any more than you would if you were reading a book and shouldn't feel painful (although can sometimes be mildly uncomfortable).
Prolonged practice not recommended!
Step 1:
Hold the stereo card in one hand at arms length, placed directly in your line of vision.
Step 2:
Position a finger (other hand) roughly halfway between the card and your eyes.
Two ghost fingers will appear over the image of the card.
Step 3:
Keeping your eyes focused on the card, maneuver each ghost finger to fall
in the center of either left or right stereo image.
Step 4:
Switch your focus from the card to your finger.
Three out-of-focus figures will now appear where there were only two on the card.
Step 5:
Remove your finger and refocus your eyes on the center of the three figures without changing the degree your eyes are crossed.
This is the tricky bit where your brain will attempt to adjust the angle of your eyes as it normally would.
It can take a few minutes or repeated attempts for your eyes to become comfortable focusing in this way.