This goes back to the days when I had an old Olympus Pen FT 1/2 frame SLR. I got the idea from the old Viewmasters where you put in a circular card which contains 2 images of multiple subjects. You look through two eyepieces to see both images in 3d. I used to take two shots using slide film (as they were 1/2 frame it didn't cost any more) and then view them using a couple of Agfa Guppi viewers. Bingo. 3d slides
I then saw that you could do the same thing with a couple of prints and just use your eyes and brain to focus on each image separately to give the same effect.
Now with digital I just do it on the screen.
To take the photos I just move the viewpoint a couple of inches across (the distance your eyes are apart) and take the 2 shots. Some work better than others and sometimes (as in the case of the snake shot above) you can just fluke a 3d.
When I was taking the snake photos (teh hand held ones anyway) I was moving around a lot taking lots of shots. The two you see above were taken moments apart from a slightly different position.
Hope that helps explain it.
Here is another.
