There are lots of ways to make a backing, if you are going to send it out to a LA, it makes it so much easier on the LA if you make the backing at least 8" longer and 8" wider, then if there is any shrinkage when quilting there will be enough fabric. Also if it is squared up it is easier and faster for the LA to get your quilt on the quilter. I charge extra if I have to square up a backing..
If I have any fabric leftover from the quilt I try to use it in the backing. But the best way for me to make a backing is to measure the length, plus 8", then double that amount. Take one length of fabric plus 8" cut cut off the selvages, then measure the width of that fabric, subtract that from the measurement of the width of the quilt, divide that number by 2, then with the second length of fabric, trim off the selvage and cut two pieces of fabric and sew one piece to each side of the first length. Clear as mud, huh. You can also google how to make a quilt backing and it will explain it better than I just did.