All you need is a pair of scissors, super glue, a bag and a piece of fabric.
Basically, you just have to cut your fabric into the size you want, and then glue it on the bag's flap. I had to fasten the fabric with my sewing machine as well, because apparently, my super glue wasn't all that super. And voliá:
Your bag now looks like this! A really easy DIY, perfect to do on boring sundays when you have no reasonable things to spend your time on.