Think of it in terms of what you already know, which is knitting. Look at the piece of knitting you want to put the crocheted edging on, and think to yourself, "Now where would I stick my knitting needle in here if I was going to pick up stitches?" And then that's where you put the crochet hook. Slip knot on the hook, stick it through the edge of the knit piece, yo the hook, pull it back through, yo, pull through both loops on the hook. Voila!