I lay the square on the background fabric and mark the diagonal on that square (not the background). Then I sew just a thread or two off the line (toward the edge - not the center). When I sew right on the line I don't seem to have enough to press back. I also do not trim through both layers when the square is attached. I only trim the triangle half of the background suare. That way when I press it back and it is off by a little, I stil have the original size of the background rectangle.