All quilt backing and batting should be squared up and pressed, and need to be at least 6" wider and 6" longer than the quilt top.
No bed sheets for backing. The fabric is too dense, and in turn it prevents needle penetration causing skipped stitches and thread breaks.
Label the quilt top and back top if necessary with a note paper marked "TOP" and secure the note with a safety pin to keep it attached to the fabric.
You might want to consider choosing a backing that will blend with the color of the top thread. I prefer to use the same color in both the top and bobbin thread, as some thread will show through if two very different colors are used.
If seaming backing, please leave the selvages on the top and bottom as it provides a more stable surface to pin to the leaders. If you are piecing the backing and have a choice, run seams horizontally and not vertically. This rolls onto the machine easier and prevents unnecessary stretching and puckering. Pieced backs are fine, but keep in mind that it's very difficult to center a back on a longarm machine, and I can't guarantee that it will be exact.