What I have done in the past is to make all the side borders one length and then the top/bottoms the same length allowing lots of extra length because you lose so much in the mitering process. Sew all borders together beginning/ending 1/4" from the edge. Then fold your quilt in half diagonally (right sides together). Line up your mitered borders, matching seams where necessary. Draw a line at a 45 degree angle this should be your stitching line. After sewing that line, check that your borders and corner are flat before trimming your sewn border seam. Adjust as necessary and then repeat the same process for the remaining corners. Hope this makes sense for you.