OK, I found it...
Quarter square triangles are cut using the finished size plus 1 1/4 inches.
Then cut in half on the diagonals both ways.... leaving the straight of grain on the long edge...
So,
For your big ones this is what I think...
Since your blocks are 12 inches and the triange spans two of them,
you would cut your square 24 7/8 inches and cut in half on the diagonal giving the big corner triangles.
The little ones are also based on this measurement, I think,, so
cut the square 25 1/4 and cut in half both ways giving you four triangles from each square with the straight of grain on the long edge.
I would do it with freezer paper first to make sure it fits..
Oh yeah, you need to measure your quilt blocks to make sure they ended up 12 inches as planned...
Judy