How big do you need the backing? Anyway, I will usually buy regular yardage which is 40-44 inches wide. I will cut three equal lengths the length I need. I will usually do not buy fabric that needs to be matched. I leave the salvages on. I line up the selvages and stitch a wide enough seam (usually about 1.5 inches) so I can trim the selvages off before quilting, it is easier to line up the fabrics with the selvages on. I do pin every 6 inches or so. When stitched, I trim off the selvages for the interior seams leaving the two outside selvages on (these can be useful for attaching the backing to the leaders of the longarm. I usually press the seams to one side. I will mount the backing, so the seams run parallel to the rollers. I do have a 12-foot frame (144 inches so can usually mount a king on) I end up with about a 1/2-inch seam and I use a walking foot as it feeds the fabric more evenly.