For many quilts, it's necessary to piece the backing to make it large enough.
I always use a .375" to .5" seam for backing.
I always trim the selvedge edge of the fabric, not the whole selvedge, but the very edge. Years ago I saw a tip to do this. Because the selvedge is woven tighter than the rest of the fabric, it can shrink differently, causing puckers.
I always press the seam open, there isn't as much of a bump when quilting.
The backing can be pieced with two lengths of fabric placed side by side. The quilt top can be placed with the length of the top with the length of the backing, or the length of the top across the width of the backing.
If the backing is a directional print, it probably won't look good with the length of the quilt across the width of the backing--you don't want flowers growing sideways or animals laying down! And there are times when you must have the length of the top with the length of the backing because of size.
If the backing fabric isn't directional, I like to leave the backing in one piece, open the fabric up (not folded as it comes off the bolt), bring the same selvedge edge right sides together, with the cut edges even. I'll try to explain that:
Let's say you have 4 yards of backing. Unfold the fabric. This gives a single layer of fabric that is approximately 40" wide by 4 yards long. Now fold it in half, right sides together, with the right (or left, doesn't matter) selvedge edge together. This gives you a double layer of fabric 40" wide by 2 yards long, with the cut ends of the fabric even.
Now sew a wider than normal seam along the edge of the fabric layers, backstitching at the start of the seam. So far it is just one folded piece, not cutting it into two halves yet. After sewing all the way down to an inch or so before the fold, snip into the fold about an inch, then sew to the end, again backstitching. I put the needle down in the seam allowance, and match the other unsewn selvedge edge, working my way down to the fold. Then I cut along the fold over to the snip at the seam. Now you have one piece of backing about 80" wide by 72" long, which is the size to run the length of the quilt top across the width of the backing.
Before pressing the seam open, trim away that tight woven edge.
Another way to piece a backing is with a diagonal seam. This works if you have fabric you want to use, but it's not long enough. I first heard of this method, probably from a Simply Quilts episode on HGTV with John Flynn. Here is his method. There are videos on YouTube also.
But what about that fabric you want to use, but even with diagonally piecing, it's not enough. Well, you can piece together leftovers from the quilt top. I did that with this backing, AND I HAVE NONE OF THE FABRIC LEFT!!!! I can't throw fabric away, even if it's not much, so using it all up is FANTASTIC!!
The fabric on the left and right is fabric I bought for backing, the center pieced portion is the leftovers.