I have pieced many backing. You need to measure your top width and length. Write them down. Then measure your backing width and length. Write them down. These number should tell you how much backing to leave on the outside before lining up the side of the quilt. This should help you center the top on the backing. There really isn't any easy way. Just try to get the top center on the pieced backing the best you can do. I do assume that you backing is at least 6" larger that the top in both width and length.
Good Luck