Picking up Stitches for a Sock Gusset
Once you've done whatever heel turn you need for your sock, you have to set your needles up again so that you're working in the round so you can shape the part of the sock known as the gusset.
To do that, you need to be on the right side of the work. If the last row of your heel was a purl row, knit across the remaining heel stitches.
Now you'll find yourself at the edge of the heel looking down the side of the heel flap. To get back over to the leg stitches, you need to pick up stitches along the edge of the heel flap.
Some people like to pick up all the stitches and then knit across them; some do what's more correctly called "pick up and knit," where you pick up one stitch, work it, then pick up the next stitch.
Whichever way you choose to do it, it will be pretty easy because you slipped the first stitch of every row when you were working your heel flap. Just put your needle into that long stitch and knit it. You can pick up one or both sides of the stitch, just make sure you do it consistently down the row.
Continue Picking up Stitches
Whether you pick up stitches and then knit them or pick up and knit them one at a time, you'll need to work consistently all the way back to the leg stitches that have been sitting on their needle or needles the whole time you were working on the heel.
Some people like to pick up an extra stitch at the top of the heel to help prevent a hole where the leg joins the foot. You can or not depending on your preference.
Work all of the stitches onto the same needle that the heel stitches were on.
Knit the Leg Stitches
Now you've made it to the leg stitches that have been waiting so patiently all this time. Knit them. Or work them in whatever pattern you were using before they went on hold.
If they weren't already all on one needle, make sure they are now. You'll need that extra needle in a minute.
Finishing Picking up Stitches
With an empty knitting needle, pick up (or pick up and knit) the stitches along the side of the heel flap on this side of the work. Try to pick up the same number of stitches on each side, that is, if you picked up an extra stitch on the first side, do so on the second.
The twist on this side is that, when you get back to the needle with the heel stitches on it, you should knit half of the stitches that were left when you finished your heel turn onto this needle. Unless the pattern states otherwise after the heel turn the end of the round is typically thought to be at the center back of the leg, so working those few stitches again and putting them on the needle with the heel flap stitches will help you remember where the end of the round is.
Once you have the stitches set up in this way you're ready to start working in the round again and decreasing the gusset stitches down the foot.