Usually when I am adding a border to a quilt I just add the border along the edges. Mitring the corners definitely does give an extra something to your quilt.
Whichever method you use to mitre the corners of your quilt, you will need to cut the border longer than the edge of the quilt.
Cutting the border strips for mitring corners
The quilt block shown is 10.1/2″ wide and the border is 2.1/2″ so I have cut the border strip 14.1/2″ long. That’s 10″ for the quilt block and 2″ extra each side. Find the middle of both the border and the quilt block by folding in half and pin the border along the first edge.
Sewing the border strips to mitre the corners
With right sides together and using a 1/4″ seam, sew the border to the top of the quilt block. Begin sewing 1/4″ from the edge of the quilt block and finish sewing 1/4″ from the edge of the quilt block. Bakcstitch at each end so that the stitching doesn’t come loose. Repeat with the second border strip.
The second method for mitring corners
Fold one of the border strips down so that it lies right sides together against the quilt and the other border strip. Turn down the seam allowance across the unsewn part of the quilt border strip as shown.
While holding the seam allowance down, fold the corner of the quilt border strip across and down to form a triangle. Turn the border strip up and pin the diagonal created to the border strip beneath it.
Beginning at the corner of the quilt block, slipstitch along the diagonal to hold the two border strips together along the mitre.
I have to admit that I don’t find this as easy or as neat a method as the first method of mitring corners that I have shown.