(Submitted by Marje Rhine, American Quilter magazine pattern editor)
I used to have a lot of trouble accurately applying mitered borders. It seemed more often than not I would end up with pointy or sometimes rounded borders. This was because I could not accurately cut the angles of the borders when they were already attached to the quilt. I also sometimes had wavy borders because most mitered border techniques do not have you cut your border to exactly fit the quilt. Here is my now-preferred method for cutting mitered borders.
Measure the quilt from side to side. Measure the width of the border. Tip: If your quilt has several mitered borders, sew the border strips together first. Make sure your length of border is long enough. To determine this, take the width of the quilt as measured, add 2 times the width of the border, and then add 2″. In my example below, the quilt measures 36″ from side to side and my border width is 4″. My length of border must be at least 46″.
36″ + (2 x 4″) + 2″ = 46″
Starting precisely at that mark, cut the border strip at a 45° angle. Make sure this cut goes in the opposite direction from the previous cut. Cut 2 borders this way—one for the top and one for the bottom of the quilt.
Measure and cut the side borders in the same manner before sewing any border to the quilt.
Now center and pin a border to the quilt. Use alignment pins to make sure that the intersections of the 1/4″ seam allowances on the border match the intersections of the seam allowances on the quilt. Stitch the border to the quilt. Be sure to backstitch at each end of the seam.
Add all borders to the quilt in the same manner, then finish the mitered corners with a 1/4″ seam.
ADDENDUM: There was a question about why I subtract 1/4″ from the measurement before cutting. Here is a diagram that shows why the diagonal for the cut piece is started at 1/4″ less than the measured quilt width. Notice how because of the angle and seam allowances the the lower raw edge of the border is shorter than the raw edge of the quilt. I must admit that I often cannot remember if I add or subtract 1/4″ to get the length to cut. For large quilts I often start the cut at exactly the width of the quilt then ease in that extra 1/4″ as I go.