Get somewhat close to the image, perfectly perpendicular, perfectly still, and cross your eyes so you get two images that overlap. Line those up on both axes, and try to look around at different parts of the image without moving your head or un-aligning the overlapped images. And try to focus on the background, not the actual foreground images. I have to see the secret image with my peripheral vision. That's basically what works every time for me. It's exactly what VR headsets do except the headset lenses cross your eyes for you, no effort needed.