I’m adding an extra round of sc (5sc in 3sc corner) so that I can use the join as you go technique.

For the border:

Rnd 1 (with black) sc with 3sc in corners. For the valleys where the squares met when joined, I did a hdc, dc, hdc to even out.

Rnd 2 (with yellow) dc around with 3dc in corners.

Rnd 3 (with black) sc around with 3sc in corners.

Rnd 4 (with black) after joining round 3, chain 1, sc in same stitch, skip sc, 5dc in next sc, skip sc, slip stitch in next sc, skip next sc; repeat from around.