They aren't really a lot of directions on the cornerstones, but in the very first part of the reveal clue she says (numbers still edited out)
"From blue fabrics cut XX squares to be used as cornerstones between the sashings. I chose a multitude from my 1 1/2’’ squares box already cut. You can go with one piece of yardage, or make it as scrappy as you wish."
edited: Glad to see you already got the answer and that you seem to be doing better with the project today.
I didn't quite make yesterday but I will be done with the extra blocks today/shortly!