When I make a big quilt, I make it in sections--could be anywhere from 2 to 6 sections--depending on the size. I join the sections together after quilting. Anyhow--what I do is cut a generous backing. Then I put the backing, batt and quilt on the dining room table. I make sure the quilt is centered on the batt and backing. Next I put a bandaid on my left index finger--that way, I don't have to put one on later on a sore finger--aha! I have a long wooden slat that I keep concealed in a closet corner. I use it to schmush the quilt flat by running it firmly over the surface. And I slip my largest cutting mat under the quilt. Then I safety pin the surface, about 3 or 4 inches apart. I shift the quilt around until it is all pinned nicely. I peek at the back to be sure that I didn't get any big creases. Sometimes just removing a pin or two is necessary. And I'm off to the races (as they say.) I'm too arthritic to do any stuff on the floor, anyhow. Also, I can do one section at a time, so it is not as exhausting for me. When quilting, leave room at the edges where it will be seamed together later.
To join the sections--never pin them together--it's too easy to loose a pin in the batt. Well, maybe a safety pin or two. I used to baste the front seams together, but I realized that since I was sewing with a needle and thread, why not just hand sew the fronts together? It's just using smaller stitches than the basting. So that's what I do. And then I turn the quilt over and trim excess batting. I use my tiny craft iron to iron down that seam. And I hand hem the back together. Lastly, I quilt over the seam.