2. Now, cross your eyes and aim to overlap both images.
3. Draw the rest of the fucking owl.
Seriously. Ever since my physics teacher in high school tried to get the class interested in stereograms, everyone and every article I see talking about it treat "crossing your eyes" as an atomic, trivial step. It isn't. I for one have no first clue how to do it, it's not a distinct operation I know how to perform. Perhaps this is because I am nearsighted and wear glasses.
Still, I wish articles like these focused on explaining how to do the whole cross-eye thing, because once you master that, everything else becomes instantly self-apparent and doesn't need further explanation (I know because I did manage to accidentally cross my eyes once or twice while looking at a stereogram, so I know how the effect looks like).
EDIT: FWIW, I compensate by using another trick for diffing documents with Mark I Eyeball - get them printed on separate pieces of paper, put one on top of the other, and hold in front of you with some bright light behind you (Sun, or your phone's flashlight, will do). Not as good as crossing your eyes, but something I can reliably do.