I do like the others have suggested. I tape the backing to the floor. I have at times, taped the top to the floor, only because there were some little puckers in areas that wouldn't come out. I would tape the top down, pull it tight and those puckers seemed to disappear.
I'd spray the top, lay the batting (warm and natural), then spray that and lay the backing....or vice versa.
Make sure you shake the can. If your arm gets tired, give it to someone else to shake for a while. I don't use 505, can't remember the name I used, but it said to shake can for 10 min. Then spray at least 8 inches from fabric. Worked great for me.