Assuming you want a lining, cut a rectangle twice as long as you want your finished bag to be, fold in half, sew edges together. (Yes, that interior pocket is anachronistic, but darned handy!)
Next, accordion-fold your rectangle four times so that your top edges are on the same side.
Using an awl, nail or other sharp, pointy object, poke two holes all the way through the folded fabric.
Thread your cord through each end as shown above. Round cotton clothesline is an inexpensive approximation of a braided cord.
