STEP ONE: make a slipknot with the end of the ball of yarn, and put it over your thumb. Tighten, but not so much that you can't slide it over your finger with ease. The loop around your thumb should still be attached to the ball of yarn.
STEP TWO: take the yarn about two inches-or more-down from your thumb, and wrap it AROUND your thumb above the original loop. Like so.
But wait! There's more!
ENDING A CHAIN! So you think you're done making this chain, when suddenly, you realize that if you just take the loop off your thumb, all your stitches will come undone. Here's how you end it off so that that doesn't happen.
STEP ONE: Take out your scissors-any kind will do-and cut the part of the yarn that's still attached to the ball.
Pull the end through the loop your thumb's in (take your thumb out first) and pull. Trim. Voila!