Ok, so this particular pillowcase was sold as "Irregular". It was cheap, but as you can see, the side seam wasn't quite straight.
If it had been, you would just need to seam rip the long seam open and cut the opposite side open to make one long strip of fabric with the short seam up the middle.
(As it was, it took me an hour to cut it into somewhat even rectangles and then sew them back together.)
Fold the long strip in half lengthwise, right sides together. Measure 8-10 inches out from either side of the center seam, and pin from the mark to the ends.
Sew along the pinned edges. Turn right side out and try on your new shrug!
(Just an FYI, the sleeves are a bit baggy, so feel free to narrow them up a bit.)
Share and Enjoy!