Fold each strip of fabric 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 fabric 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 (Miter 1 and Miter 2).
Align the edge of a ruler with the raw edge of the top strip so the triangle’s long edge intersects the border seam in the corner. (Miter 3)
Draw along the ruler edge from the seam to the raw edge. (Miter 4)
With right sides together, match marked seam lines and pin. (Miter 7)
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. (Miter 8) Press seam open.