Most of us learn how to cross our eyes when very young. As you probably remember, it's easily done by trying to look at the bridge of your own nose. Do it now.
Did you notice that when you looked at your nose, all the objects around you split into pairs? Let's explore that a bit. While looking at the figure below, cross your eyes and observe how the white square splits and becomes two squares.
Our goal here is to control how far apart those two squares appear. You probably noticed that when you crossed your eyes, the squares went very far apart indeed. We think of eye crossing as an act of caricature, and an all-or-nothing exercise: either they're crossed or they're not. But of course that isn't true. You are simply controlling muscles that are not so different from any other muscles in your body. If you can cross your eyes to an extreme, you can also cross them a tiny little bit, or any amount in between.
If you don't have a sense of such fine control, try crossing and uncrossing your eyes several times quickly. Get familiar with the muscular sensation. For our present purpose, it might actually be useful to fatigue those muscles a little bit. Now see if you can start to cross your eyes but interrupt yourself and uncross them again. See how this makes the squares stay closer together.
With a little practice, you should be able to make the squares hover near each other, and vary the distance at will. Try to make them hang on the screen a few centimeters apart. What you perceive might be something like the animation below.
Important: Don't continue to step 2 until you feel you have mastered step 1.
Let's do the same thing as in the previous step, but starting with two squares instead of one. Being a mathematical whiz, you probably expect the two squares to split into four, right? Try it.
But you don't want four; you want three. Trust me on this. Since you learned distance control in step 1, align your four squares so that the two in the middle overlap. The "X" will help you do this with precision. What you perceive might be something like the animation below.
Repeat the previous step, but with this diagram.
Once you have made the two squares into three, pay attention to just the middle square. Let the other two loiter harmlessly in your peripheral vision. Does the "E" appear to be positioned slightly behind the other letters? If so, congratulations! You are ready for 3D crossviewing.