1.
Along the edge of the heel flap you should see a column of slipped stitches which each look like a letter 'V'.
2.
Insert your needle under both legs of the first slipped stitch, bringing the needle under the leg closest to the outer edge of the heel flap first.
3.
Loop your yarn around your needle as you would for a knit stitch, then bring your needle back through the slipped stitch, with the loop still wrapped around it.
4.
Repeat steps two and three for each slipped stitch along the edge of the heel flap.