to make them match, draw your seam allowance on each piece. then put a pin vertically through the intersection of the seam and the line you drew, checking the opposite side to make sure it also goes through that intersection. do not turn that pin to any side. now place straight pins from the piece toward the seam allowance on either side of the vertical pin. iwhen sewing, sew through the intersection of the original seam and your drawn line. i hope this makes sense
