Slip stitch joining method:
https://www.craftpassion.com/flat-slip-stitch-granny-join/

inspired by caseyplusthree’s blanket here.

Alternate squaring rounds.
For smaller corner “holes”:
HTR: yo and draw through 2 loops, yo and draw through 3 loops
Round 6; Ch 3 (counts as first htr), working in back lps only, ★ dc in next st, hdc in next st, sc in next 10 sts, hdc in next st, dc in next st, htr in next st, ch 2), htr in same st (corner made); repeat from ★ around.
Round 7: hdc around as per pattern, 3 hdc in each corner.