Hold the Mayo! Quilts come together like a sandwich. There's a top, a back and the filling, and here are some tips and tricks to handling each piece.
If you are going to have your quilt finished on a long arm machine, you will need at least 6 inches of backing fabric and batting on all sides of your top. Measure your quilt top in several places (top, middle, bottom) to get the exact size.
When you have your quilt top done, the next thing you need is the back. The back needs to be square, and cut straight of grain to allow your quilt top to lay flat after quilting. If you piece your back, remember the more pieces, the harder it will be to keep it straight of grain, and absolutely do not use any selvages. Trim them off (they shrink differently than the rest of the fabric). One nice way to buy backing is 108" wide backing.
Now that you have your top and back ready to go, you need batting, which is used as a layer of insulation between the fabrics. Batting comes in many, many kinds, weights and brands; it usually is made from cotton, polyester or wool. Some batting works better for hand quilting, some makes your quilt puffy, and some makes quilts lay nice and flat. The kind you use is your choice. Everyone has a different opinion.
If someone else will do your quilting, ask what batting they prefer. As a long arm quilter, I prefer Warm and Natural cotton batting because it is really nice, lays flat and doesn't stretch as it goes through the long arm.