Take a t-shirt and cut off the sleeves , just a the seam. don't cut the seam just right next to it.
I took a small dinner plate and cut out the top of the shirt. Then turn it in-side out.
Sew the bottom of the shirt and then turn it right side out.
Waaa-Laa You have a reusable grocery bag. a swim bag ,a soccer bag a whatever you want bag:)