I use my king size bed to lay the different layers out (I can't use the floor to do that anymore as it hurts too much :cry: ). I keep the backing piece larger on the sides of the quilt (by about 6-8") as it comes in handy when putting it on my quilting frame. That way I can quilt to the edges of the quilt without the bunge things (for keeping the sides of the quilt taunt) from getting in the way of the machine. I like to have the batting larger than the quilt top by 2" all around to allow for shrinkage that can occur when quilting. After quilting I have been able to trim the selvage from the backing so it can be used to make the binding. I have used cutting mats as squares to get the backing & batting squared up nicely. I have even found that my bed works fine to do a little ironing with the larger pieces of material. Hope this helps. I'm sure someone else might have some different ideas of how to align the different layers. Hope some of this helps!
Pam