Oh, no -- I've left you hanging far too long. Now that you've knit 36 rounds on the leg of your sock, it's time to work the heel. Place the next 18 stitches on a single needle; these will be your heel stitches. Place the other 18 stitches on a piece of yarn (use a crochet hook or a sewing-up needle to pull the yarn through the stitches and then remove the needle) so they can stay out of the way while you work the heel flap:
You will now work back and forth on 18 stitches, as follows:
Row 1: slip first stich as if to purl, bring yarn to back [this is the only
time you need to move the yarn; for the rest of the slipped stitches it
stays behind your work], k1, *slip 1 as if to purl, k1,* all the way
across. You will end with a knit stitch; if you find your last stitch should be slipped, you have goofed. Go back and try again.
Row 2: slip first stitch as if to purl; purl across until there is 1 stitch left. Bring yarn to back and knit that stitch.
Repeat rows 1 and 2 until you have worked a total of 18 rows on the heel flap. Work row 1 one more time. You are now ready to begin a purl row on the heel flap.
Row 1: slip first stitch as if to purl; p 10, p2 tog, p1.
Turn your work (that's right, in the middle of the row)
Row 2: slip first stitch as if to knit, k 5, k2 tog, k1. Turn work.
Row 3: slip first stitch as if to purl, purl over until you have 1 stitch before the gap formed by the previous p2 tog. Purl the stitch before the gap together with the stitch after the gap, purl 1. Turn work.
Row 4: slip first stitch as if to knit, knit over until you have 1 stitch before the gap formed by the previous k2 tog. Knit the stitch
before the gap together with the stitch after the gap k 1. Turn work.
Continue in this way this way until all stitches have been worked, all the way across. You should end having just worked a knit row, ending on the last stitch on the needle.
Next installment (and I won't make you wait so long): pick up stitches and knit the gusset.