Step 14: Extra stuff I didn't do
I like the raw cardboard/duct tape look that I came up with. However, you can have a more polished look to your bag. In the fabric section, they have iron-on patch compound. It comes on a roll, with a plasticy material on a kind of wax paper. The compound melts under an iron and holds materials together (like hot glue, but in sheet form).
Using the manufacturer's instructions, iron the material onto your fabric, allow to cool, then peel the paper off. Place the fabric on the cardboard, patch material touching the cardboard/duct tape. Situate the fabric how you want it before you iron it on - you may want to fold the edges of the fabric underneath so they don't show. If you fold it, you will need some more compound on the underside of the fold so it will stick to the bag as well.
If that sound too convoluted to follow, I will make another guide to doing that. Good luck!