1. When you cross your eyes, gradually let them return to uncrossed. Try to do it as slow as possible. Along the way, try to line up any structures that you see in the image that are repeated from left/right half.

2. Once you are able to hold a cross-eyed gaze long enough with lined up left/right half, slowly move your eyes between different features near the middle. Your eyes will naturally want to start to focus and match up pieces.

3. Don't be too far or too close to the image; they are usually easily viewed from comfortable distances. If the image is too big, make it smaller. It's usually easier smaller.

4. Initially, when you cross your eyes, or look through the image, it will likely be blurry. This is because your brain naturally associates accomodation and convergence with also changing focus. You'll learn to decouple those things and you will more quickly be able to go from focusing on the 2D image to crossing it without changing focus much.

There's a whole bunch on this site: https://www.magiceye.com/stwkdisp.htm

What worked for me: I’m not “crossing my eyes.” Trying to cross my eyes precisely enough to get the two images to overlap was impossible.

Instead I’m starting by crossing my eyes until a third out of focus image starts to appear between the other two images, then I immediately abandon trying to cross my eyes and instead focus on bringing that third image into focus like it’s “floating” in the room at a different point in space than the page its on.

Apparently the circuits in my brain and eyes are good at this. As soon as I can get the “third image” to show up by crossing my eyes, I can “look” at it and my eyes snap perfectly into position.

After about 5 minutes of practicing I can focus on the third image in about a second from the time I begin to cross my eyes.

On the third image, the differences between the two images “shimmer” like a holographic foil on a playing card.