Your situation may vary slightly from what is shown in this tutorial, but the idea is the same. You want to pick up stitches along the sides of the heel flap. I don't know if you slipped stitches at the edges of your flap, if not you won't be picking up under both legs of the slipped stitch vees, but otherwise the same.
It sounds like you were supposed to cut the yarn and rejoin it to where you pick up stitches. In this tutorial you are already there. This is part of Silver's Sock Class. LINK