This technique works particularly well with variegated yarns and the garter border makes it easier to pick up stitches on your heel flap.
Row 1 (RS): sl1, k3(4), *k1, sl1. Rep from * until 4(5) sts rem. K to end of row.
Row 2 (WS): sl1, k3(4), p until 4(5) sts rem. K to end of row.
Row 3: sl1, k3(4), *sl1, k1. Rep from * until 4(5) sts rem. K to end of row.
Row 4: sl1, k3(4). P until 4(5) sts rem. K to end of row.
Repeat rows 1-4 until desired length, then work heel turn as written. One of the advantages of the garter border is that you can count garter ridges and have your heel flaps the same length.
TTFN!