here goes:
cut out your square. whatever size you like. i don't know what the size
ratio will be. you'll have to test that.
fold up/down on center.
fold side to side on center.
fold the points in to the the center. make sure the folds all meet.
the folding is done.
draw a straight line from center side to center side, all around that
inside diamond 1/4" in, toward the center.
the measuring is finished.
cut on that 1/4" line.
two suggestions: try it first on paper before you try it on fabric, then try it
on one layer of fabric before you stack fabrics.
do not move the fabric while cutting. keep those cut
away triangle where they are.
you should get the same result with out the mat instructions.