A friend recently shared that one of her quilting goals for 2021 is to master the mitered borders. I have found that making multiple mitered borders is just as easy as a single border, so I thought I would share my process with you.
Step 1: Sew the inner border pieces to the outer border pieces using a scant ¼ inch seam. The top/bottom borders should be the width of the quilt + twice the width of the border +1 inch. For example, if the quilt is 50 inches wide, the inner border is 1 inch wide and the outer border is 4 inches wide the length of the top/bottom borders should be 60 inches: 50 + (2 x 1 inch) + (2 x 4 inches) + 1 inch = 50+2+8+1. The right/left side borders should be the length of the quilt + twice the width of the border + 1 inch.
Press the seam allowance of two strips toward the inner border (place on sides of quilt) and two strips towards the outer border (place on top and bottom of quilt).
Find the center of each border unit by folding it in half. Mark with a pin. For top/bottom borders, measure 1/2 the width of the quilt on either side of the center mark and pin. Then fold the end pin to the center pin to find the quarter mark and mark with pins. You should have five pins. For example, if your side is 25 inches the pins would be placed like this:

Step 2: Pin the quilt top and borders.
Find the center of each side, the top and bottom top and mark with a pin. Find the midway point between the center and end on either side and mark with a pin, as above. Align the center and quarters of the border with the centers of the top and pin at marks. Align end point of top with outer pins on border strip and pin. Pin rest of borders to top.
You should have border fabric left on either end of the quilt top.
IMPORTANT – Mark the quilt top on the wrong side with a dot 1/4 inch from each corner. Start sewing at one 1/4 inch mark and stop stitching at the mark ¼ inch before the end of the top. Backstitch at beginning and end of seam one or two stitches. Sew all four border pieces to top leaving ¼ inch unsewn on each strip at every corner.

Press seams toward the quilt top.
Step 3: Making the miter.

Lay the quilt on ironing board with the finished side down (A). Fold quilt diagonally as shown (B) so the back side of the quilt is showing (C). NOTE: the A1, A2, B1 and B marks on the illustration are for reference only, you do not need to add these marks to your quilt. Line up the borders from the two sides aligning the marks 1/4 inch from the edge of the quilt top (C).
Align the border strips. Mark the border at a 45 degree angle, as shown (C). Pin, matching creased lines and seams. Slowly machine baste from backstitching point to outer edge of quilt.
Press seam open and check accuracy with 45-degree rule. If it is accurate, sew over basting stitch , from backstitching point to outer edge of the quilt. Remember to backstitch one or two stitches at beginning and end of this seam because it will be the outer edge of the quilt. Remove basting stitch. Trim the seam allowance to approximate ¼ inch. Repeat this process for the other three corners.

That’s it – a double mitered border!