Somewhere along the line, I'd removed the zipper for another project, so I just sewed the back seam shut and centered the piece for the main body of the bag along that seam. You can just barely see the hem tape near the top of this picture (and Isabella's rear end).
I have no idea if I put this together the way they intended. Well, I'm almost sure I didn't, because the sleeves and shoulders looked completely wrong at first, and I ended up redoing them. But it came out just the way I wanted, so I'm happy.
Now here's the funny part: I thought I'd post a tutorial with more explanations and pictures, but in a few places I ended up scrapping the directions and doing it my own way. So do I still post it or not?