Cutting the strip or border to focus on a particular element of the stripe is commonly referred to as "fussy cutting".
To fussy cut your border, you will need to know the measurement of the unfinished width of the border. Using a chalk wheel, mark a line of chalk down the center of the motif you wish to use as the center of your striped border. Divide the unfinished width measurement of the border in half. My diagram uses a 6 ½" unfinished border measurement as an example.
Half of 6 ½" is 3 ¼".
Measure 3 ¼" away from the center chalk line on both sides and cut on those lines. Before cutting, you can use this chalk method to determine the number of repeats you can derive from the width of your striped fabric.
It won't be long before I will be showing you my next line of fabric with Henry Glass Fabrics - Sentimental Stitches! And it has a beautiful stripe, too! Can't wait to show you!
Until next time!