I would use my 6 1/2" X 24" ruler. I would find the center of the braid and line it up from the top to bottom of ruler half the width that you want it, and trim, and then turn the braid over and find the middle point again and trim the other side the same way. My pattern does not have sashing strips, but I would guess they will nest together and with the use of a stylus, ease them together, so each seam meets evenly.