Oh, so the image was a tortoise upside down? Now I see it

For me, the secret is to relax my eyesight a bit. Kinda try to ignore the image and think of something else. "Space out" while staring at it.... and suddenly boom, you're in

Focus your eyes as if you're looking at a more distant object. You know how if you look at something a little further away and hold your finger in front of your eye it doubles? With a magic eye, the image is repetitive, and you're trying to get the doubles to overlap and "lock" together, tricking your eyes into thinking they've focused correctly. The repetition is varied in such away that it makes the illusion of a 3d object.

The kind of weird thing is that you can achieve the same effect with any repeated pattern - floor tiles, a fence post, etc. But there's no underlying illusiory image, so you just see basically the same thing but everything else is focused wrong.

I used to love paper books of these as a kid. I was taught to start with my nose practically touching the page and slowly backing my face away without changing my focus. Eventually I could do it without doing so (and the skill never faded!) but it's the best way to get started