Take a sleeveless shirt you don't wear.
Turn it inside out.
Sew the hems together.
And there you have it.
I cut off the bottom of mine to use for a headband, and then I flipped the shirt 1/4th of the way around so the side seams were matched. That way the handles of the bag will be in the right spot.
I loved the color of the shirt, but I didn't feel like I could wear it much. I'll use it more now. :)