Step 6: Cut, Sew, Attach the Strap
Cut two strips 3" wide, and the length that you determined in step one. Sew them right sides together, 1/2" from edge on three sides, leaving one short end open. Clip corners on the closed end.
Turn the strap right side out through the open end, as if inverting a tube sock. This is very tedious; I'm sorry. Tuck the raw edges in and topstitch to finish.
Lay the strap on the back of the bag and pin in place. Try it on and adjust as needed. For the cream bag, I had 16"-13"-30" (below the bag, across the bag, above the bag), and for the green bag I had 22"-12"-35".
When you've got the strap pinned where you like, stitch a rectangle to secure it to the bag, making sure to pull the front of the bag out of the way so you don't stitch it shut. If you want to be fancy, put your strap thread on top and your lining thread on the bobbin, and it will look super slick. I wish I'd thought of that before I finished. If you do this, be sure to test your tension on a scrap so you don't end up with loops of one thread showing up on the other side.