Here's how I did it: with the example images on screen (~10cm wide), stare at it from around 25cm away. Cross your eyes until the images overlap. To cross your eyes you should keep the distance to the screen around the same but actively tries to move the focus. Do so until the image overlaps and merge. Now adjust the distance to the screen very slowly while keeping the focus so the merged image don't split, until the merged image become sharp. The merged image may never become as sharp as viewed directly, but that's enough for us. Now notice the spots where the image appears semi-transparent/flickering/bloty. You may not be able to tell in detail what the differences exactly are, but you should be able to tell the relative locations. Note that depending on the focusing, nearby spots may merge as one. Also I found the "hard mode" challenge easier than the "easy mode" in the sense that the difference stands out more clearly.