Most of your backings (unless you are making a queen-sized or larger quilt) will require a single seam in the backing. Here's the quick way I go about preparing and sewing that seam.
You know you'll have to cut your yardage in half. Don't try to measure it; just fold it in half and place a pin in the fold.
The next step is to sew the two halves together along the long sides. You'll need to remove the selvages before you begin sewing this seam. But before you do that, here's something to think about.
If your backing is directional, you'll want to make sure that the two halves are facing the same direction once you sew the seam. Without wrangling yards of fabric, here's an easy way to make sure that both pieces are oriented the same way.
Have you ever really looked at selvages on fabric? Usually, one selvage has printed information on it, and the other one doesn't. If you remove the printed selvage from one half of backing and the plain selvage from the other, you are guaranteed that your print will be in the same direction once your seam is sewn. Of course, the exception is batiks, woven plaids and solids. You'll just have to pay attention to your pattern orientation on those.
Make a snip with a pair of scissors parallel to the selvage.
Place your two halves right sides together, and sew them together using a 1/2" seam. (Note: If you don't use pins, you will probably end up with one edge being 1/2" longer than the other when you get to the end of the seam. Don't panic - it's okay because you added extra to the length or width when you calculated your yardage amount. This just emphasizes the fact that fabric feeds at different rates through the feeds dogs. This is why you NEVER should sew on borders without pinning!) Press the seam open and you're good to go!
If you are piecing a back with 3 seams, you will have to measure your yardage into thirds before you cut it. Work with the selvages the same way; remove the selvages from piece 1 and piece 2 and seam together. Press the seam. Now, remove the selvages from the other side of piece 2 and piece 3 and seam together. By doing the sewing in steps, you'll guarantee that the print on all 3 panels is oriented in the same direction.