This tutorial shows how to make a yarn puffball (a.k.a. pom-pon). I believe this method is more sophisticated than the other methods I've found online. It produces more durable puffs that can be attached temporarily or permanently.
Step 1: Items you'll need:
1. Yarn (between 150 and 200 yards)
2. 30" Crochet thread (I think this is size 8 but I'm not positive. For this application, the bigger the better, we need it to be strong.)
3. Empty soda can
4. Darning needle