Make yourself a manila folder with a piece of fine sandpaper, just glue the sandpaper down onto the folder and when not in use the folder will protect the sandpaper. This, to me, is the best way to mark lines for half-square and quarter square triangles, as the sandpaper stabilizes the fabric for easier marking with a mechanical pencil. If you have a 1/4" foot on your sewing machine, use that, put the edge of the foot just over the line to sew a scant 1/4" accurately. hope this helps.