You will either use a knitting needle or a crochet hook, slip either through the top edge of the side of your sock, pick up the yarn from the inside/back and pull a loop of yarn through.
Keep that new stitch on your needle (or if you are using a crochet hook to pick up stitches place that loop on one of your two empty needles.
Continue to pick up stitches along the edge moving away from the heel cup as you go, toward those stitches you have on the stitch holder.
Pick up stitches evenly and pick up 13-15 stitches, however many fit nicely in there for you. I usually end up with 15. The important thing is to remember how many you pick up, and pick up the same amount on the other side of this sock and also on your next sock.
After you pick up the stitches on the one side you will knit the stitches from your needle holder. You may need to first slip them onto a needle, or you may be able to knit them straight from the stitch holder.Now you have just one side without stitches. Pick up the same amount of stitches on this side as you did on the parallel side. Again using either the empty needle or a crochet hook.
What you will do at this point is move the stitches from needle one (the heel cup) to needle 2 and 4. So divide the stitches on your heel cup in 1/2 (9) and you will knit 9 stitches onto needle 4.
The slip the remaining 9 stitches from your heel cup onto needle 2 (which will now be needle 1).