The gusset of a sock gets the leg stitches and the heel stitches back together and shapes the sock so it comfortably fits around the heel and the top of the foot.
To begin, take an empty needle and pick up and knit six stitches along the side of the heel flap. The stitches you should pick up are easy to find because we slipped stitches along the edge of the heel flap.
Next, use another needle to knit across the 10 stitches of the leg that have been hanging out.
Pick up another empty needle and pick up and knit six stitches along this side of the heel flap, plus three of the stitches from the heel itself. Slip the last three stitches of the heel onto the needle with the first stitches you picked up (so you have an empty needle again) and knit those three stitches, along with the first stitches you picked up.
Now the stitches from the leg are considered needle one, the next needle is needle 2, and the needle you just finished knitting is needle 3.
To shape the gusset, work the following two rows:
Row one: Knit straight across the first needle. On the second needle, knit 1, slip, slip, knit, and knit to the end of the needle. On the third needle, knit to within 3 stitches of the end of the row, knit two together, and knit the last stitch.
Row 2: Knit straight around with no decreasing.
Repeat these two rows until you have five stitches on each of the second and third needles; 20 stitches total.