Watch this knitting instructional video to learn how to do a knitting provisional cast-on. A provisional cast-on is usually worked in a contrasting waste yarn so the cast-on stitches can be easily removed later. The live stitches are then recovered so you can knit down from that point without the joining line you'd have if you picked up stitches along the edge. There are several provisional cast-ons; this is the one I like the best, because I find it to be the easiest to work. You'll need a crochet hook in an appropriate size for your working yarn and needles, and a length of waste yarn.
Start by making a slip knot loop on the crochet hook. (This loop does not become a stitch.) Now, holding the knitting needle in your left hand, and the crochet hook in your right hand, bring the yarn behind the needle.
Bring the crochet hook in front of the needle and pick up the yarn. Pull a loop through the loop on the crochet hook. You have cast on one stitch.
Take the yarn back behind the knitting needle and repeat this process until you have the desired number of stitches cast on.
Break the yarn and pull a large loop through the last loop on the crochet hook. You don't want to tie off, because this is the end from which you will unravel the chain to recapture stitches.
When you are ready to recapture the live stitches at the cast on edge, gently pull out the chain, placing each stitch on a needle.
Hope this knitting tutorial video helps you with your provisional cast on.