Before You Begin
Picking up gusset stitches is a lot easier if you do some preparation when you're knitting the heel flap of your sock. Just slip the first stitch of every row of the flap.
Slipping that first stitch creates a neat column of elongated stitches along the flap edges. It also decreases the number of edge stitches. You end up with one edge stitch for every two rows.
Picking Up Gusset Stitches
Generally speaking, one stitch is picked up along the edge of the heel for every two rows of knitting. So, if you've slipped the first stitch of every heel flap row, you just pick up one new stitch in every edge stitch. You can either work through both legs of the edge stitch or just the front leg.
To pick up new stitches, insert your right needle tip through one or both legs of the edge stitch, wrap your yarn around the needle knitwise and pull it through.
See it in Action
(Just in case you need to review, here's how to knit a band heel.)
If you'd like to make the Switchback Socks shown in the tutorial, the pattern is available in my shop.