http://amotejer.wordpress.com/2012/07/14/como-ovillar-la-lana-con-la-mano-how-to-wind-yarn-into-a-ball-by-hand/
These are the written instructions to accompany the photos above (also cut and pasted):
Hi, everyone! I come back today with a tutorial I loved! Maybe you are already mechanized and have a yarn swift or ball winder, then you might think is this lady crazy? 21st century and she winds ball by hand? Or maybe you are just like me, you own both but you find it so complicated to pull them out of the closet, put them together, work with them, then put them away again, that you just prefer doing it by hand. And when you are knitting, you reproach yourself for not using them because the ball of yarn is dancing around everywhere. And not to mention a baby going after it all the time! It´s impossible to knit! Well, now you understand why this tutorial caught up my attention. And even though you may figure it out just looking at the pictures, here are a few explanations. I really hope you find it useful! I will try it next time I start knitting!
1) Tie a slip knot in the end of your yarn.
2) Tighten this slip knot on your winding tool. I like to pull this slip knot tight in the hook of the crochet hook, or high up toward the end of the knitting needle, to keep it out of the way.
3) Hold your yarn against the winding tool with your thumb, and wrap it around the winding tool several times near the end as shown.
4) Continue to wrap yarn around these first clump of wound yarn, criss-crossing as you work from time to time as shown. This should feel just like winding a regular wrapped ball of yarn, only this ball of yarn has a winding tool poking out of it.
5) Continue in this fashion until all your yarn is wrapped, then tuck the end into the ball under a few strands to secure it.
6) Still holding that first bit of yarn between your thumb and the winding tool, gently ease the tool out of the ball.
7) All done! The end with the slip knot in it will now be the start of your next project, and yarn will pull from the center of the ball with ease