Some people have a terrible time with mitering.
Let me make it easy for you. Measure length and width of quilt top. Use those measurements and add at least eight inches on each end of borders for mitering. After sewing the inside border to the outside border, overlap the extra as show in the picture above.#1 picture
#2. Picture shows folding under the top border to form a miter. Be sure that the wrong side of the top piece is even with underneath piece at the top and at the bottom. Pin down the fold and press with a hot dry iron
#3 Picture shows unpinning after pressing and gently folding the top piece back to see crease line made by pressing. Draw over the crease with a dark pen. Repin across the crease and line, and stitch starting at the top. Stitch with a basting stitch, in the event that you haven't gotten the miter perfectly in the center of the corner. When sure of that, restitch with a regular tight stitch.
#4 Picture shows the miter perfectly sewn and perfectly at the center of the corner.
#5 Picture shows the underside of the miter with the seam allowance and excess borders materials removed. Press with a hot iron and DO NOT OPEN SEAM! Press to one side!