Next time heavily starch the backing before layering. Starch stabilizes the fabric so it is much less likely to pucker. What you can do with the sandwich you have is lay it out on a large flat sheet, spray starch the back working from the outside edges towards center to minimize overspray, let dry (a fan speeds drying), then repeat spray starch layers several more times. You can do the same to the top. This will help prevent puckers and tucks when you quilt.