To create mitered corners on a quilt border, first determine the measurement of the border to cut the fabric. Find the length of the quilt side, and add it to the product of the width of the inner border multiplied by two. Add 6 inches to account for fabric needed to create the mitered edge; this is the side borders measurement. Substitute the length of the top into the equation to find the length of the top and bottom border.
Center the fabric along each portion of the quilt and pin to create seams 1/4 inch from the edge of the fabric. Sew into place, backstitching on the edges to secure the stitching. Place the quilt on an ironing board with the design facing up, and allow each overlapping border to fold into itself at a 45-degree angle. Iron the fold to create a distinct crease.
Starting at the upper-right corner, fold the intersection of the top borders together. Fold the raw edges evenly with both the backstitched corner and the ironed creases. Pin to secure, and then stitch the length of the iron crease. Backstitch close to the border seam. Open the seam, and use a square ruler to check that the angle is precisely 45 degrees; if so, trim any excess fabric to 1/4 inch from the seam. Repeat with the other three corners to complete the quilt's mitered look.