And here it is with the folds ironed in to make sewing simpler:
Here is where the strap is attached after sewing the long sides together but before the folds ar sewn.
And here is what the strap looks like when sewn into the side seams:
BTW - the cord used for this strap was made from scrap yarn and floss made into a twisted cord. If you have never tried this technique, here is a site with a video demo.