How to Miter Borders on a Quilt
When our instructions specify mitered borders, the cutting instructions include the extra length needed to miter corners. However, if you'd like to substitute a mitered border for a standard straight border, cut strips the width (but not the length) specified. For side border strip length, measure through quilt center from top to bottom, then add twice the border width plus 6". For example, if your quilt measures 40" long and your border is 5" wide, you would cut your side border strips 56" long (40" + 10" + 6"). For top and bottom border strip length, measure through quilt center from side to side, then add twice the border width plus 6".
If you're adding multiple borders, first join border strips for each side into a single border unit. Fold each border strip in half crosswise; press lightly to mark centers. Fold quilt center in half in each direction and press lightly to mark center of each edge. Pin a border strip to quilt center edge, matching the center marks and allowing excess border strip to extend beyond the corner edges. Sew together, beginning and ending the seam 1/4" from the quilt center's corners. Repeat to sew border strips to remaining edges. Press all seams toward border.
To miter each corner, lap one border strip over the other. Align the edge of a 90-degree right triangle with the raw edge of the top strip so the triangle's long edge intersects the border seam in the corner. Draw along the triangle edge from the seam out to the raw edge. Place the bottom border strip on top and repeat the marking process. With right sides together, match marked seam lines and pin. Beginning at the inside corner, sew together the strips, stitching exactly on the marked lines. Check the right side to see that the corner lies flat. Trim excess fabric, leaving 1/4" seam allowance. Press seam open.