Take the cake of yarn, pull the center pull and the outside strand, and join them. I was using cotton this time, so I braided the ends together. Then I just do the long-tail cast on for as many stitches as I need, cut off the strand from the outside of the ball/cake/whatever, and my working yarn is then from the center. No measuring, wondering, or getting to the end without enough yarn. I love it. Depending on whatever kind of join you use, braiding, spitting (for wool) or Russian Join will all work. It's totally cool. Takes the anxiety out of casting on lots of stitches.