This technique is the goal that you will want to finally achieve if you need to go through the learning process of the other techniques. If you are one of those characters that used to voluntarily cross your eyes just for fun then this technique should be relatively easy for you.
Focus your eyes on the space between the two photos. Slowly start to cross your eyes. Notice that there is a third image that starts to grow between the other two. Continue crossing your eyes until the center image is the same size as the other two. You will now have 3 images. The center image should be fully 3 dimensional.
Position your face about 24 inches from the screen. Place your thumb between your face and the screen with the top of your thumb at the bottom of the pictures and in line with the space between the pictures. Focus your eyes on your thumb. Think about the picture. Without changing your focus from your thumb you should be aware of three images. Slowly move your thumb back and forth between your face and the screen until the middle image is very close to the same size as the other two images. In some cases it may be necessary for you to slightly tilt your head side to side so that the bottom edges of all three images line up. Your challenge now is to shift your focus from your thumb to the center image which will be fully three dimensionable if done correctly. Most people need to practice this many times. Shifting the focus is a challenge. Don't get discouraged. You can do it and the final result will be amazing and exciting.
Some people will find making a training aid window helpful. Take a piece of printer paper, fold it in half one direction and then fold it again in half the other direction. Now cut off the folded corner by 1/2 inch each direction. Unfold the paper and you should have about a 1 inch square hole in the center. (After reading the rest of these instructions scroll the image pair to the center of the screen.) Position your face about 24 inches from the screen. Place the paper with the square hole in it about 10 to 12 inches in front of your face. Close your left eye and position the paper so that you can see only the left image through the hole. Now while holding the paper steady in that position, open your left eye and close your right eye. If you see only the right image through the hole that is good. But if you don't then you need to reposition the paper so that you see only the right image through the hole. The repositioning of the paper should be done by moving the paper back and forth between your face and the screen. NOT LEFT AND RIGHT. Continue alternating between viewing through the hole with your left and right eyes one at a time until you achieve the position of the paper such that both eyes see their respective images without moving the paper. Now open both eyes and look at the image in the square hole. It should be fully 3 dimensionable. You may need to rock your head a little bit side to side to find the perfect focus. You are actually looking cross eyed through the hole in the paper with the crossing point being at the paper.
Here is another image to try.
Go to the 3D photography page to see more images.