Sew a border to your quilt top leaving a length of fabric hanging out each end longer than the width of the border. Start sewing ¼” in from the edge, backstitch to secure and then sew to the other end, stopping ¼” from the end and backstitch. Press seam toward the border.
Attach a second border the same way.
Use a square ruler and place it in the corner to line up the loose ends of the border. Trim off excess fabric.
Fold your top diagonally to the corner. Start sewing where the borders meet the quilt, backstitch and sew diagonally to the corner of the border.
Trim off excess fabric and press the seam open.
You can also use this technique for multiple borders. Let us say that you have to add 3 mitered borders to the top. First sew all three borders together as one piece so they are longer than the final border. Then following the above instructions, treat them as one solid border except the seams in the borders must be carefully matched. This is a lot faster and easier that sewing on each border individually.