I do what Cari does... I measure a piece of fabric the (smaller) width of my quilt x the width of the fabric (40-44 inches wide). So I would need 2 times the width plus about 6 inches that I cut in half. Then I trim and sew the selvedge side to either another selvedge side or my favorite is to piece a width of the quilt with fabric scraps from the front. So that I don't have to match the fabric on the back and the seam isnt' as noticeable across.
here's one I did that way:
https://www.pinterest.com/pin/118782508899891458/
Often that way I can also decide to use two different fabrics altogether! Here's one such example:
https://www.pinterest.com/pin/118782508903439498/
The only time it doesn't work is if you use directional fabric!