if you have blocks in the center then - measure your border with the center of the quilt. Make sure both side borders are the same. Then mark down the border piece - say each 12" if that is the size of the blocks. Then pin where the seam in the top should fall on the border. Carefully sew easing in any fabric that has stretched. If you do this by sewing on the border side then the feed dogs should ease in the fullness. It can be done without stay stithing, fusing etc but careful pinning and sewing is the solution when I do it. You can't just cut the borders and sew without carefully preparing each layer.