There is a video on this site under Videos/cast ons, but I could never get it to work for me.
Do you crochet at all? Because the easiest way for me to do a provisional cast on is to take a crochet hook and crochet slip sts around my needle. Another method is to make a chain with 6-8 sts more than what your project needs cast on, then turn the chain over to the back or bumpy side and with your knitting needle pick up a st in each of those bumps until you have the number needed for your cast on.
There are also a lot of good videos on places like You Tube, so you might search for "provisional cast on" and see what you can find.