Typically, flap and gusset style heels are not my favorite to knit, in part because I don't enjoy picking up stitches on the sides of the heel flap. However, I discovered that it can be easier to determine where new stitches should be picked up if you work the first and last 1-3 stitches of each heel flap row in garter stitch.
Traditionally, when you work a heel flap, the first stitch of every row is slipped. This leaves you with half as many edge stitches as there are rows. For example if your flap is 32 rows long, you would have 16 sts along each edge of the flap. Then you pick up one new stitch in each of the edge stitches.
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