It's time to work the gusset. For the gusset, stitches are picked up both sides of the sock before continuing to work the foot. On the heel flap the first stitches slipped at the beginning of every row will make it easier to pick up the stitches along the edge for the gusset.
Step 6: Working a gusset step-by-step
To start. With the needle that is holding the shaped heel. Working down the heel flap on the left-hand side. Pick up and knit 19sts through the slipped edge stitch being careful not to allow a hole to form at the base of the heel flap. If necessary, pick up a few extra stitches judging which loops to go through to stop this from happening.
Move the stitches. Holding the needles together move the core down on the front needle pushing the stitches up onto the needle.
Get your needle ready. Then pull the stitches on the back needle down the core so the needle is in the position to work the first stitch on the front needle, knitting across the 30 stitches on the top of the foot.
Next. Holding the needles together move the core down on the front needle pushing the stitches up onto the needle, then being careful not to allow a hole to form at base of gusset. If necessary, pick up a few extra stitches at this point judging which loops to go through to prevent any gaps. Pick up and knit 19 sts up the other side of the heel again through the slipped edge stitch. Then across the 18sts of the heel.
You did it! You have now created the start of the Gusset, with the upper part of the sock on one needle and both sides of the heel flap and heel turn on the other needle for the back of the sock. You will now have 86sts