often when having multiple borders to miter we sew the borders together then miter the whole thing at once- much easier than mitering each one individually.
sew all of your border strips together- then add to each side of the quilt- stopping at 1/4" point of each corner-with the extra fabric (flapping) on the corners- then you can miter each corner around the quilt- 4 miters instead of 8 or 12- or how ever many borders you plan to add x4 corners.