When I used to put my back on the floor, I would smooth it out and tape the edges down with masking/ painters tape to the floor. You want it taut but not stretched or the the back will rebound from the stretch when you take it off the floor. I put pins through the tape that is on the fabric edge so it doesn't peel off until I am done. Then I smooth out the batt and top and start to bast. You put a marble under the backing so that you can roll it along as you bast so that you have room to put the pins in or the needle for thread basting.
I use Hobbs 80/20 fusible most of the time now and the sandwich has to be ironed instead of pins. I do this on the old carpet in the basement.