Such a striking square! Had a few problems; mostly me probably as others have completed square without saying anything! There was some error, re instructions and count. Below are notes which resulted in my finished square. Wrote the designer; so watch for her errata, as her changes will be the best and likely different than mine here.

Rnd 8: Ch 2 (counts as first hdc), (2 hdc in next st, hdc in next 2) 15 times, 2 hdc in next st, hdc in next, sl st in 2nd ch of beg ch-2 to join (64 st).
Note: Corrected to add # of times to work sts each side, and terminology example. (Pattern will say 2 hdc instead of hdc in next 2 sts, for instance).

Rnd 17: With color D, join with sc in any st, sc around with (sc, ch 1, sc) in each corner ch-1 sp, cut and join to first sc. (124 sc – 31 sts per side)

Rnd 18: (Note: This is the round I had a hard time interpreting; this is my re-write, in keeping with what I think the designer meant). With color A, join with sc in sc before any ch-1 corner sp, ch 1, sc in corner ch-1 sp, ch 1, sc in sc right after ch-1 sp (corner made), * (ch 1, sk sc, sc in next sc) 14 times, ch 1, work corner as before, * rep between * and * four times, ch 1, sl st to first sc to join; finish off. (68 sc, 68 ch – 17 sc and 17 ch-1 sp per side)

Rnd 19: (See Rnd 18 Note). With color D, join with sc in any non-corner sc, * dc in front of the ch-1 sp into sk sc below (on rnd 17), sc in next rnd 18 sc across, up to the ch-1 sp just before the corner sc, work corner: dc in front of the ch-1 sp into the rnd 17 corner ch-1 sp (the same ch-1 sp that rnd 18 corner sc is in), sc in rnd 18 corner sc, dc in front of the ch-1 sp (right after the corner sc) into the rnd 17 corner ch-1 sp again, sc in next rnd 18 sc, * rep between * and * 3 times, sc in next rnd 18 sc, dc in front of ch-1 sp in sk sc of rnd 17 up to beg sc, sl st to beg sc to join; finish off. (136 sts – 34 sts per side)

Rnd 20: Final count should be (152 sts - 38 sts per side)

Rnd 21: Final count should be (164 sts - 41 sts per side) Note: The pattern seems to pull the corners off a bit, as a natural by-product of the design. The designer took this into consideration by working the non-standard corner count. It is not an error.

MY NOTES:
SS yarn says it’s worsted, but it’s more sport weight. I use it with size G hook to make 9” squares.
Rnd 20: Did dc instead of hdc in corners to “square up” a bit.
Rnd 21: Did sc around with hdc in corners. Did 3 sc decreases evenly on each side to pull make the sides flatter as they wanted to round out a bit.