Originally Posted by QM
My instructions from my wonderful teacher were to 1. sew to 1/4" from the edge on each side. 2. Fold the body of the quilt on the diagonal, so that one border is exactly over the other. 3. Use your 45* mark on your ruler to cut both borders at once, being sure to leave your seam allowance. 4. Pin carefully. 5. Sew from the inside out.
I follow similar rules, but I a) leave extra fabric on all 4 borders until after the miter is complete and b) use the 45º angle line on my ruler to mark the sewing line, not the cutting line. Then I baste on the line from the corner out, check it for accuracy, stitch it (or rip and try again), trim the seam, and it's done. By leaving the extra fabric, you get one or more do-overs for free and by basting the line first, if you need to rip, it's just basting. ;)