Place a dark square on your light background square, right sides together, line up the corners. Sew, on drawn line. Fold square towards edge of block and press, voila, triangle at the corner of your unit. If you wish you can trim out the extra layers to remove bulk, depending on how you are going to use the unit, having three layers of fabric can make your seams quite bulky. Before you trim, check to make sure that the triangle edge matches the outside edge of your square, if not leave background layer to use as a guide for piecing your block together. (Pic3a-f)