It was easy as pie, all I did was take out the existing insole, which was simply glued on, and traced it on a piece of paper. I used that as a template for cutting out my fabric. I put the original insole back in as is since the glue that held it in before was still able to keep it on the shoe.
I used a paintbrush to spread Elmer's glue on the sole, and put in the fabric. I used the wrong end of the paintbrush to smooth the fabric, and let the glue dry. That's it :)