I started by tracing the bag's pattern onto paper (although I think you could use a regular plastic bag to design a pattern as well).:
As you can see, the edges of the bag are a little wide so they can fold in, like a plastic bag. This helps give the bag some volume:
I laid the pattern on the fold so all I had to do was stitch up the sides, finish the edges, and attach a little ribbon with snaps! With the help of an iron and some folding skills, Voila! You have the picture at the top of this post. These make fantastic gifts and stocking stuffers, and they are so useful! I carry two of them in my purse wherever I go!