I used to do this on a large table, but I have since discovered that working on the floor gives me a much better result. I would never crawl around on the floor to pin a quilt (because it would just kill my knees and my back), but this is fast and relatively easy.
I am fortunate to have a large space on my bedroom floor that will easily accomodate a queen sized quilt. I start by folding the backing in half lengthwise, selvage to selvage. Then I place it on the floor with the fold along the edge of one of the floor boards and tape it in place with masking tape. I'll also secure the opposite corners to keep it from shifting as I'm laying out the batting.
Next, I fold the batting in half and place fold on top of fold on the backing, then do the same with the top.
I use a couple of rulers to square up the edge for the first cut, then trim all the way around the quilt, leaving a 3" - 4" border, which is usually what the machine quilter asks for.
A cautionary note here, make sure you know where the end of that cutting mat is as you're going along. You don't want to roll off the end of the mat and cut your floor!
Here is my quilt all squared up and ready to go off to the machine quilter.
What do you do with the left over pieces of backing and batting? The fabric will get trimmed down to a suitable size
and added to the strip drawer. The smallest scraps of batting are used to wipe oil off machines, collect the tiniest scraps and threads from my cutting mat, dusting whatever needs to be dusted in my sewing room, and whatever else I can think of to do with them.
I'll use the larger pieces as batting in smaller quilts. I used to whipstitch together these smaller sections of batting to make a larger, more useable piece, but I've since discovered that once I have a small quilt loaded on my frame, I can simply butt one of these smaller pieces up against another and keep quilting. I've washed the quilts afterwards, and those batting pieces seem to stay in place just fine. I would not do this, however, for a quilt that would be quilted on a domestic machine or one that would be hand quilted. Been there, done that, and it didn't work well for me at all.