All you need is a tennis ball and a knife. Any sharp knife will work, but a shorter blade is preferable so that you're less at risk of cutting yourself.
Step 2: Cut!
Take the trusty blade and cut the ball along the seam. Be careful here since you'll be turning the ball around a lot. I made lots of small cuts here. You won't save too much time here by speeding things up anyway.
Step 3: Done!
Presto! A pair of jar openers. The rubber inside grips nicely on metal lids, making it a lot easier to open a jar up.
Step 4: Decorate it
Add a couple of dots and your little jar opener has some more personality! No effect on the effectiveness, but it's more fun.