I have a few thousand extra selvages laying around so they seemed like a natural choice. I also had some lime binding leftover, so I used it for the top and bottom.
I simply layered a backing, and batting and sewed these 'quilt as you go' style about four inches wide and about ten inches long. I then wrapped it around the cup, and trimmed away the excess, leaving enough for a quarter inch seam allowance.
I made the binding on the top and bottom edge just like I would on any quilt. It was really easy and didn't take long at all. If you want to make one, I would suggest cutting a piece of paper 4 inches wide, and wrapping it around the cup. Trim the paper to the best width and length for your cup, not forgetting seam allowance, and use that as your pattern.
Also, if you have a reusable cup, once it's washed, put it back in the cupholder in your car, so you will have it with you next time you visit Starbucks! I'm still trying to remember to do this, but I'm getting better!
Enjoy every sip!