I made my bag fully lined, so it adds a couple extra steps. If you do not want to line your bag, then start out by making it like I made my gift bags, and hop back here to insert the boning.
First, I cut two large rectangles of fabric and placed them right sides together, stitching up and around the whole outside, leaving a small opening to pull the fabric through. Then, I turned it out and topstitched, so there would be no raw edges on the final product.
Then I folded the project in half and stitched all the way up two sides.
Once I had the boning in, I cut it to fit and stitched it together. Now, this is a tricky part. You have to cut the plastic in your boning shorter than the fabric, so that you can stitch it shut. DO NOT TRY TO STITCH THE PLASTIC. YOU WILL BREAK YOUR NEEDLE! You will have to wiggle the boning out of the casing a little bit, in order to get it up under the needle of your machine. However, once you are done, you will be able to just slide the boning back in place.
Ta-da! Now there is no excuse for a messy car!