For mirror image quarter square triangles, cut two contrasting squares, draw ONE diagonal - - sew on both sides of the drawn line, (1/4 inch seam) - - cut apart on the drawn line, iron with the dark behind the seam.
Cut another colored square that matches the size of your HST - - draw a diagonal line on that square, right sides together with the HST ... sew 1/4 inch seam on both sides of the drawn line, cut apart on the drawn line and viola! Two mirror imaged quarter square triangles, and I don't have a clue as to what you use them for, even though they are in that one square as points in a star ... sorry about the camera <wave>