To make mitered corners on a quilt, first attach the border or edging fabric to the main body of the quilt. Next, sew the corners of the border fabric together at a 45-degree angle. This a relatively complex process that requires exact measurements and precise calculations.
First, determine how much material you need for each side of the quilt border. To determine this amount, take the measurement of the quilt side, and add the width of the border multiplied by two. Add 6 inches to this sum to obtain the final measurement needed. For example, a quilt measuring 36 inches by 60 inches with a 2-inch border needs 46 inches of material on two sides and 70 inches of material on the other two sides. This is calculated 36 plus 4 plus 6 inches.
Next, measure and cut the border fabric. Attach the borders to the quilt by sewing the material from the center to the corners. Stop sewing 1/4 inch from each corner, and backstitch the seam to reinforce it. Now, fold the quilt in half on the diagonal. The quilt forms a triangle at this point. Line up the border fabric with right sides together. Using a ruler, mark a line 1/4 inch from the top fold of the quilt across the border fabric. Starting from the side seams that are 1/4 inch away from the corners, sew the border fabric together along the line you marked. Repeat for each of the four corners. Trim the excess fabric, and press the seams to finish.