There's a quilt shop in Texas called The Fat Quarter Shop that does amazing YouTube videos on a variety of quilting topics. Recently, they partnered with Moda and did a quilt-a-long online called "The Ultimate Beginner Quilt" series. Along with this series, they made companion videos to help you every step of the way. One of the videos was called "Piece your Backing Together". A couple comments on the video from me, from a longarm quilting perspective:
1) I do love it when my clients press their backings before bringing them in. Most of you know that I do press everything before I load your quilt on my frame, but I always appreciate it that it's been pressed first and all the hard wrinkles are gone before I receive the backing;
2) I do need 4-5" of backing all the way around the quilt top. So if your quilt is 40"x50", then I would need the quilt backing to be a minimum of 50"x60". (5" extra on each side").
3) You do not need to trim any extra backing off for me. If it's too big to load, I'll trim it. I always prefer to have options when I'm loading your quilt on my frame and it's nice to have the extra.
4) I personally seam a quilt with 1/4" seam and press the seam to one side. Cathy and Kathy of Sonflower Quilts always told me that the backing seam has a lot of stress on it, so if you press it open, you have only one little thread holding your back together. I have no trouble longarming a quilt with a seam pressed this way. It's not too bulky.
5) PLEASE - IMPORTANT - If you're joinining the two pieces of your backing together with the selvages together (which she does not do in the video), Make the seam 1/2" and cut the seam down to remove the selvages. If you've pressed the seam to one side and left the selvages, then the seam is too bulky for my longarm. I can go through the bulk, but then you run the risk of having a skipped stitch or two on your quilt top.
Hope this gives you some good information, my friends. Have a great day!