So you've finished the leg of your sock. Now it's time to work the heel flap. First, divide the stitches in half.
You'll put one half on a stitch holder and work the heel on the other half. Divide the stitches so the working yarn is coming out of the last stitch of the group you are going to put on the holder, ready to work the first heel stitch. Some people use a knitting needle as their holder; others use those holders that look like a giant safety pin. However, because those are straight and not flexible, and actually do weigh a little bit, they have a tendency to stretch out your corner stitches, leaving unsightly holes later on. So -- use a length of contrasting yarn. Thread it through your 18 (20) instep stitches, and remove those from the needle. You now have half your stitches on a strand of yarn, and the other half on a single needle. The working yarn is ready and waiting.
(Be careful, on the first stitch of the heel flap, not to pull the yarn so tight that you close up the last stitch on your holder -- you will regret it later).
For the heel flap, you knit back and forth, as follows:
row 1: slip 1, knit 1, slip 1, knit 1, continuing in this pattern all the way across and ending knit 1. (If you did not end with a knit stitch, you goofed -- undo and fix it.)
row 2: slip first stitch as if to purl, purl the whole row until just 1 stitch is left, move yarn to back, knit last stitch.
Repeat those two rows until you have worked 17(19) rows. You will end having just worked a knit row.
This part is just plain magic! It's not complicated, but it's hard to describe without a lot of words. If you haven't done this before, don't think too hard -- just follow the directions step by step.
Row 1: slip first stitch as if to purl, purl 10(11), purl 2 together, purl 1. Now turn your work -- yes, it's true, you have not finished the row. This is called a short row, and it's a way to build curves into your knitting.
Row 2: slip first stitch as if to knit, knit 5, knit 2 together, knit 1. Turn your work.
Row 3 (and all subsequent purl-side rows): slip first stitch as if to purl, purl across until there is 1 stitch left before the gap that was formed when you turned the work before. Purl that stitch together with the stitch on the other side of the gap, purl 1. Turn your work.
Row 4 (and all subsequent knit-side rows): slip first stitch as if to knit, knit over until you are 1 stitch before the gap formed when you turned the work before. Knit that stitch together with the stitch on the other side of the gap, knit 1. Turn work.
Continue working according to this pattern until you have used up all the stitches. You should finish at the end of a knit row, with 10(12) stitches on your needle. If you have extra stitches, or don't end at the end of a knit row, something has gone wrong and -- I regret to inform you -- you have to rip out and work back.
You have formed a nifty little pocket, just right to cradle the bottom of the heel.
Now you have to get all the other stitches back on the needles and knit the gusset, a triangular section that accommodates the ankle.
Pick up stitches for gusset
Take one of your empty needles and, starting on the side of the heel flap where your working yarn is waiting, pick up stitches along one side of the heel flap. You will see a convenient line of V-shaped stitches along the side of the flap (my apologies -- the yarn I'm using for this sock is fuzzier than I realized, and not ideal for this illustration).
I prefer to pick up the stitches by sliding my new needle under both legs of that V, then "knitting" through it with the working yarn. Pick up a new stitch for each V along that side edge, all the way to the end of the flap.
Now it's time to put your 18(20) instep stitches back on a needle. Slide the needle back in along the path marked by your holder yarn. I leave that yarn in place -- it makes an easy marker to count from to match the second sock to the first, or if it's necessary to rip back. You may prefer to remove it. Either way is OK. Knit across those 18(20) instep stitches.
Now it's time to pick up stitches along the second side of the heel flap. Follow the same technique you used on the first side, making sure you have the same number of stitches here as on the first side. (Sometimes you have to "invent" one to make this happen.)
Now all 4 of your needles are full of stitches. What to do? With the needle that just finished picking up your heel flap stitches, knit 5(6) stitches from the next needle. Slide the other 5(6) stitches onto the next needle. You are now at the exact center of the back of the heel. You have 18(20) stitches on your instep needle, and 15(16) stitches on each of the side needles. (Sometimes I get an extra stitch, for 17 -- as long as both sides are the same, that's fine.)
(People sometimes like to continue the ribbing down the foot; for thick socks like these, I think it's more comfortable to do the foot in plain stockinette, so that's how these directions continue.)
Work one round plain.
Now you will gradually decrease the number of stitches on those side needles, until you're back to 9(10) on each. The 18(20) stitches on the instep needle remain constant. Work as follows:
Decrease round: on needle 1 (that's the one that goes from the center heel up to the instep needle) knit until you have 3 stitches left, k2 together, k1. Knit across the 18(20) instep stitches. On needle 3 (that's the one that goes from the instep needle back to the center of the heel) k1, ssk [slip 1 as if to knit, slip 1 as if to knit, insert lefthand needle from left to right through front of those 2 stitches and knit them together), knit to end of needle.
Work 1 round plain.
Repeat these two rounds until each of the side needles has 9(10) stitches.
Next installment: the foot