The cast off edge where it has the rows of K1, P1 ribbing will be around the baby's ankle (it may be turned down in a cuff after it is sewn and in use). The sides you are holding in your hands in your last picture will make the back seam and the curled part at the bottom will be on the sole of the foot. I believe the little shaped part in the middle will house the toe.
I think you will want to use mattress stitch to sew the seam. It may change a little as you go the whole length (I'm not sure) if the relationship of the stitches to each other changes. Here is a link to how to do mattress stitch on this site. LINK
Scroll down to "Finishing" and select "mattress stitch". This is a LINK
to an on-line knitting book There are quite a few chapters on seams. See chapter 17, 18, 19 and you might need 21 if you have any of that type. I was going to share one more, but I keep trying to go there and it times out before it makes it.