For anyone who wants to learn this, try this way using your finger as a helper.
Put the images in front of your eyes.
Bring your finger between your face and the image at almost middle of the distance.
Now look at your finger.
Move your finger back and forth and notice the background (where your picture is)
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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It worked on everyone I have tried to teach. You may always need help of your finger or a tip of a pencil or whatever. But it's lot easier to get those images to merge this way.
My eyes seem to immediately refocus as soon as my finger moves away no matter how many times I try this. Before I move my finger away, everything in my peripheral vision is too blurry to be useful.
I've always been able to merge pictures by focusing into the distance, and I thought that's how everyone did it. So I found confusing all the talk of crossing eyes. I tried your instructions and, within 30 seconds, was able to see the cat image by crossing my eyes.
The cross eyed method seems more amenable to different image sizes. With my regular method, I can't merge images if they're too large (unless I step back).
Put the images in front of your eyes.
Bring your finger between your face and the image at almost middle of the distance.
Now look at your finger.
Move your finger back and forth and notice the background (where your picture is)
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.
--
It worked on everyone I have tried to teach. You may always need help of your finger or a tip of a pencil or whatever. But it's lot easier to get those images to merge this way.