One of the most wonderful parts of this bag is how little preparation is involved:

Cut Fabric

Cut two 16-inch squares out of each fabric and the interfacing.

Mark Fabric

Using chalk mark a line 3-inches from the bottom and both sides on the right side of each piece of fabric. Then mark 1-inch from the top on all the pieces, also on the right side. Make sure you have your fabric oriented in the way you want your bag to go. In this case I want the stripes to run up and down. These markings will be your guide to ironing the bag into it's tidy shape after you've sewn.

In case it's hard to see the chalk lines on the above fabric here's a picture of the marked  Shot Cotton.