Hook size: 4.00mm
Main stitch (Harlequin) pattern: here
Starting chain count: 136+1 (multiples of 8 + 1) I used a bigger hook for the starting chain (I often do to balance the fabric's stretch amount) but I think I could have used the 4.00mm here and it would have looked a bit nicer.
Main part: 16.75 repetitions of the 4-row design in the Harlequin stitch pattern (see link above). Drop the last quarter of the 4-row design at the very end so that the blanket ends match.
Sc around the whole edge, sl st to join.
Hdc around, sl st to join.
*Ch 1, hdc in back loop only* repeat from *to* until edging is desired width.
Using the bigger hook again, sl st in back loop only, all the way around, sl st to join.
Note: You'll have to play around when you get to the corners. Increase as you turn, but be careful that it still lies flat once you start down the next side. You can change the increase count for a new row, but be consistent on all 4 corners.