Do this with newspaper first: 12 1/2" sq for black. Measure or fold to get half. Take ruler and put the 1/4" mark on the middle top mark and the edge of the ruler on the bottom right/left corners and mark line. The orange is half the size plus seams: 6 1/4" (make it 6 1/2" to allow for any seam difference, by 12 1/2" or 13" for allowance (so 6 1/2 x 13 should be ok). Cut to opposite corners thru middle. Sew paper together. If it fits or is close, you have done it right and you can square up to 12 1/2". Check your measurements after squaring. Should be 12 1/2" on bottom (black) and 6 1/4" on each side at top. You will have a lapover at top, but that disappears when you add your sashing or whatever goes around the square. Hope I explained it right.