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Slowly lower your gaze until both of your eyes are focused on the tip of your nose. This may cause a bit of an eye strain, since you're probably not used to using your eye muscles this way. Though you won't be able to see it yourself, your eyes should now be crossed. But this is no fun so far -- since you're looking down, no one else can see that your eyes are crossed. If you really want to master eye crossing, then you should have a friend there to watch you do it, so you can make sure you're really doing it successfully. If you look up at your friend and the friend says, "Ew!" or makes a gross face, then chances are that yeah, you've successfully crossed your eyes. People say you can try taking a picture of yourself in the cross-eyed position to see if you've done it successfully, but that just takes way too much coordination.
Focus both your eyes on the tip of your nose. Get help from a friend.