So here is what I did (could call this a basic tutorial but it is mostly common sense)
I cut a length of fabric 10cm wide which allows 1cm either side for turning. The length you need will depend on the width of your towel - take that and add an extra 2cm for turning. Prepare your fabric by turning over raw edges toward the wrong side of the fabric (don't forget to wash your fabric beforehand!)
Pin into position. You could use interfacing but it really isn't necessary - I didn't have any issues with the fabric moving while sewing.
Stitch around the outside edges as close as you can
Easy peasy - all done!