I lay my border pieces on top of the quilt center across and up and down, Lightly run my hand over the border to get to the same length as the top, cut it and do the same with other end. I then pin top in center, then pin halfway to the center pin on each end, do the same with the border, pin together and hold at each pin and sew, match up the ends and sew to the end. Sometimes this will take up a bit of fabric without puckering anything. I piece on diagonal and it never stretches. I have also had to attach a bit of fabric to lengthen tha border. Not noticeable until you look really close. I hope this helps and gives you an idea of possible fixes.