Important: If you plan on taking your quilt to a long arm quilter for quilting, ask their requirements before making your own.
If your quilt top is less than 40" in either direction, you can simply cut the backing from straight from the bolt of fabric.
If your top is more than 40" in either direction, you can either purchase wider backing fabric or seam fabric together to create your backing fabric.
To make a seamed backing, cut two lengths of fabric the same size. You will sew them back together along the selvedge edge for the least amount of work. Remove selvedge before seaming and press the seam open when finished.
I find the easiest way to determine the lengths to cut is to draw a simple diagram. Draw the shape of your quilt adding the width and length measurements, then add 4" all around it. Add the amounts up to determine the size. For example: 4" + quilt length or quilt width + 4".
Now, assume you have 40" of usable fabric (from selvedge to selvedge) and determine which direction your backing fits best - either vertically or horizontally - and use the other direction to cut the length of the two pieces.
Always give your quilt backing a good press and remove any loose threads before layering.