Go up to the top of this page, to the tabs. Run your mouse over "view videos" and to the right, there will be "casting off". Click on that and then watch that video. You should be all set.
Just remember that when you're done casting off, you will have a stitch left on your right needle. That throws off some people's counts. You'll see what I mean when you do it.
When you finish casting off, your yarn will be at the tip of your right needle, right where you need it to continue on.