How to tie a shell obi from Make Your Own Japanese Clothes by John Marshall, a very funkily illustrated book I found through Ichiroya's interview with the author. Honestly some of the ideas in his book are too 80s fashion outrage for me, but this is the best simple illustration I could find. For tying a bow, simply drape the folded end you see in #1 over the doll's shoulder so that it's longer and when you get to #3 tie a bow and position it at the top of the obi, and slide the obi around to the back. (Or just start from the back in the first place, since you're dressing a doll).
And here is a very nice YouTube vid on how to wear Yukata for girls (guys don't have side openings, but it's the same up to the two ties. Men only do the lower, hip bone tie. It shows the full layered traditional dressing. There is also an extra instruction on tying that bow.