how I approached crossing: first practice just crossing your eyes and observing how every object has two images in this case and when you slowly “uncross”, they merge back into one. you can use anything in your surroundings.

then for the stereogram you do the same, observe the out of focus edges of the left and right pictures, then slowly uncross until left and right image occupy the same spot as though they were the same object. now its out of focus, but one (ok, actually three, because there were two, you “doubled” that by crossing, then merged two of them. but ignore the other two and focus on the merged pair)

sometimes you will merge images of the same picture, in this case you are just back at your normal vision, repeat :)

then you try to keep them overlapped and focus the vision, try to “believe” that you are really looking at a single object.