Here you can see my line of stitching down the sock and the way I've turned it to round off the foot. Don't worry if you can't get the curve perfect; mine turn out pretty angular actually. Once you've finished the foot, take a few backstitches to secure the seam and break the thread. Then go back to the heel and do the same thing again on the other side of your pen line. This next picture shows the end result pretty well.