Is the backing all one piece, not plaid or stripes? If so then it won't matter if it isn't exactly straight as long as you have enough extra around the edge before quilting. If you are using a back with a seam. stipes or plaid and you want to be sure to get everything lined up well, mark the center of the length and width of both the quilt top and back. I use pins at these points and I can feel them to line things up properly even through the batt. Are you using pins to baste the sandwich together? If so it is best to smooth out and anchor the back to a flat surface before adding the batt and top. Some people use the binder clips on a table to do that but there are many different methods to do this. A marble place under the back will give you enough space to get the pin into the sandwich and back up to close it. Roll the marble to the next spot and so on until done. Some people use 505 basting spray so they don't have to pin. I like to use Hobbs 80/20 fusible batt that you iron. There are lots of posts on Qb on all the different methods. Good luck!