Before you buy the backing, check with your quilter to see how much bigger she wants the backing to be. Usually about 4" to 6" on all sides. Batting will be the same. She may have batting you can buy.
When you quilt your own, you will need to stretch the backing out (I used the kitchen floor) and secure the edges (painter's tape). Lay batting on top and secure it. Spread quilt top over these and pin with curved pins. Distance should be approx. the width of your hand.