I spent about 3 hours total on this project - one making the materials and two putting it all together.
Step 1: Materials Needed
Waxed Thread (9 ft or so) - I got this from the leather section in Joann's
2 Upholstery Needles
Baseball That Needs a Makeover
Cup of Warm Water
Step 2: Remove the Insides
I had no need to start from scratch since the insides were fine, but the leather shell on this ball had seen better days. I pulled out the trusty pocket knife and started opening it up a little more until I could pry the core from the leather.