First, get a crochet hook that's compatible with your needle size. Lion Brand has a handy chart here. I used a size 6 needle, which means a G hook.
Make a slip knot around your crochet hook, just like you do around your needle when casting on.
|Hook with a slip knot.|
|Hook into the purl ridge.|
|Yarn through the slip knot, leaving the knot behind, stuck in the purl ridge.|
|A stripe in progress.|