Now, I'm sure you're familiar with
crossing your eyes! However, crossing just one of them is a bit different, but not as hard as you might think! All you'll need is two
eyes, a
finger, and..well, that's about it!
First, take your finger and place it directly in front of one of your eyes. It doesn't need to be touching or anything, just relatively close. Basically, somewhere between your nose and your eye.
Then, focus on your finger with both eyes. That is to say, the eye with the finger in front of it should be looking straight ahead, and the other one should more or less be inspecting your nose.
Then, all you have to do is take the finger away while still focusing on the place where your finger had been.
If done correctly, the eyes should remain in their previous positions, one staring at your nose, one staring off into space.
You should be able to get it on your first few tries, although you may want to see about moving your finger faster, slower, moving it away out in front of you, or the ever fun no-finger trick.