I have never used wide backing fabric so that means that I am always piecing my backing. One thing that matters to me is to have the pattern match so that is my first approach. I'll need two pieces of fabric to do this so after I make sure I have enough fabric I cut it into two pieces.
To match the pattern I take one piece of fabric and fold over the selvedge edge to the back along an easy to follow pattern repeat (ie a vein in a leaf, petal of a flower etc) to make it easier to match. I press that fold and will use it as a reference later. I then find the repeat on the other piece and lay the first piece with the pressed selvedge edge over it to find the match. Once the match is found I pin a long the folded edge just through the second piece of fabric. I then open up my first piece and with right sides together match the pressing line with the pin line in the second piece and pin the two pieces together. I then stitch along the pressing line, check to make sure the match is good and then trim my seam allowance to 1/2". I press my seam allowance open. When done correctly you will barely be able to see the seam from the right side. Now I have a piece of fabric that is big enough and looks like one piece.