When I did this for my mom's quilt, I laid the fabric out on a flat surface (floor, kitchen island, conference table at work--whatever you can find). Using a 6" x 24" ruler, I drew a chalk line all the way down the first side of the area I wanted to capture (I was cutting parallel to the selvage as I didn't want seams in this border). Once I had the first side marked all the way down the length of the fabric, I measured over the desired width and drew the second line. Once both chalk lines were in place, I could then draw the other three in the same position relative to the print across the rest of the fabric.
It took a fair amount of time (and hyperventilating!) to avoid a cutting mistake (I had just enough fabric but not enough to correct if I miscut). In the end, it was worth the effort because the flowers in the print are centered in the border's width, which was the goal.
Good luck with your border!