I finished the headboard! And I love it. Immensely. Here are the details. (And here is Part 1 if you need to go back.)
Materials for the rest of the headboard:
2 wooden planks that measure 39" x 4" x 3/4"
Deck Mate screws (from Home Depot); they each measure 1 1/4"
To figure out where to put the legs, I very unscientifically held the headboard up on the bed and put one wooden leg behind it. I marked a line with a pencil and measured down so I could mark the other leg the same. It was 16 inches from the top.
I made marks on the headboards with pencil signifying top and bottom and the 16" mark. Then I covered them both with fabric and stapled them on the back.
I laid the legs down close to the edge to keep the line of the headboard somewhat continuous when viewed from the side. You'll see what I mean.
I placed my screws and put the drill bit in the drill (see how that works?) and drilled baby drilled (love you Sarah Palin, miss you!) (Not.)
Here she is standing up all on her own. Isn't she lovely? The legs are a bit scrawny to be carrying that weight but I figure if it doesn't seem to be working, I can always add a leg (even an non-upholstered leg) in the middle.
You can attach the headboard to the bed frame right here (who knew? I didn't until my mom told me) if you want. I just left it standing on its own and it's fine so far.
My daughter doesn't care about it so much, but I love it. I will take advantage of these years when she doesn't care about decor! And here she is now, playing the way she plays in my dreams, with minimal mess and reading a book aloud to herself.
Here's the side view, you can see the leg making a continuous line of color all the way down even though the headboard stops just below the top of the mattress.