1. With pillowcase inside out, stitch the open end closed.
2. Cut off bottom third of case (which will be your strap).
3. From the strap, cut off 1/2 inch of one side and the bottom and discard.
4. Holding the open side of the bag, fold over the fabric 4 inches, making sure to turn under the raw (or cut) edge. Press and hem (be careful not to stitch through or you will seal the bag closed).
5. With two hands, hold the bag on both sides, pull out, and lay flat. (You will have a triangle on each side.)
6. Sew along the base of each triangle (about 3 inches below the corner).
7. Grab the strap piece. Unfold it with the seam facing out, so you have one long strip. Fold over the top and bottom edges about an inch and press. Fold the top and bottom edges again so they meet in the center and press. Stitch each side down the center.
8. To connect the strap to the bag, fold under the raw edges of the strap 1/4 inch. Match one edge of the strap to the seam of the bag and stitch in a square. Sew other end of strap to the bag the same way. Turn the bag inside out and enjoy!