Take an old T-shirt and turn it inside out. Draw a bag shape you like. (You can make it as small or large as you want.)
Cut through both layers so you have two identical pieces. You can leave the shirt’s hem intact or snip it off.
Stitch the two pieces together all the way around, leaving the bag’s opening untouched.
To prevent fraying, either serge or apply a narrow zigzag stitch to the edge.