Besides being very sturdy, this is also very comfortable. I love the cross-stitch effect of the slipped stitches, too.
The increases for this pattern are worked along the edges of the foot. These instructions are written for two circular needles, but this can also be used with DPNs. If working with DPNs, you may switch to the full number of needles whenever you want. Just be sure to mark the row's halfway point to separate the instep from the sole. Both are worked the same, so it doesn't matter which is which.
Finally, your gauge in heel stitch will be different than in stockinette or garter stitch. My normal gauge for this yarn and needle (size 1) is about 7 sts. and 11.5 rows per inch. In heel stitch my gauge is about 10 sts. and 16 rows per inch.
Cast on 14 stitches (or whatever number of stitches you prefer) using a Turkish Cast-on or Judy Becker's Magic Cast-on.
Row 1: *K1, S1 purl-wise.* Repeat to end.
Row 2: Instep (needle 1) - *K1, make one left-slanted increase (M1L or LLinc). K to one stitch before end of needle. Make one right-slanted increase (M1R or LRinc).* Repeat from * for sole (needle 2).
Repeat rows 1-2 until desired number of stitches is reached for sock foot.
Row 1: *S1 purl-wise, K1.* Repeat to end.
Row 2: K to end.
Row 3: *K1, S1 purl-wise.* Repeat to end.
Row 4: K to end.
Repeat these 4 rows until toe reaches desired length.