Bring your finger between your face and the image.
Now look at your finger.
Move your finger back and forth.
While doing this, notice that at a particular distance, the images in background will perfectly overlap each other.
That's your moment.
Pull out your finger and look at that image.
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Should take lot less tries to learn doing it without finger. I have taught cross eye to my siblings and cousins using this method. But if you always need finger to focus it's fine.
Put the pair of images in front of your eyes.
Bring your finger between your face and the image.
Now look at your finger.
Move your finger back and forth.
While doing this, notice that at a particular distance, the images in background will perfectly overlap each other.
That's your moment.
Pull out your finger and look at that image.
---
Should take lot less tries to learn doing it without finger. I have taught cross eye to my siblings and cousins using this method. But if you always need finger to focus it's fine.