The gusset of a sock is the area just after the heel, which is wider to accommodate the larger diameter of your foot. It is wider and then decreases down in size as your foot needs slightly less room. What we have right now looks like this:
So before we actually get to the decreases, first we have to connect the last stitch we knit at the heel to the sides of the heel flap. A closer look at the side of the heel flap shows all the stitches we slipped while knitting the flap. They look like regular knit stitches only they are twice the size because instead of knitting them on every row, they were slipped every other row. So they look like nice big V's.
What we need to do to turn those V's into actual stitches on our needles (so that we can once again knit around and around) is to "pick up" stitches. This is not too difficult. You are going to take your empty needle and insert it under the very first V.
Then wrap the yarn around as in a normal knit stitch.
Pull the wrapped yarn through as in a normal knit stitch. You have now picked up one stitch. Since you were not knitting from one needle onto another, you don't have to slip the stitch off onto the opposite needle; it just stays on the single needle.
Then move on to the next V and repeat by sliding the needle under the V, wrapping the yarn, and pulling through.
Continue picking up stitches along the entire right side of the heel flap. You should end up with around 15 stitches, although if it is a few more or less it's okay.
The next 32 stitches we had left on two needles. Just knit across those stitches normally. You don't have to do anything fancy with them. I usually knit them all from two needles onto one, but that's up to you. (Hahaha, I also switched to yellow notes here because it's more visible against the red sock.)
Now you are at the left side of the heel flap. You will work this side the same as the right side, by picking up 15 (or whatever amount you did on the other side) stitches in each of the V's along the flap.
Now you should have 15 picked up on each side, 32 on the front or top part, and 18 left from the heel.
Knit half of the heel stitches (9) onto the same needle you just picked up the left stitches with, and transfer the other half onto the next needle. That leaves 24 on the first and third needles and 32 on the second needle.
Next post will be about the gusset decreases!