Do not use an iron on polyester batting as it will smash it flat or melt it. A wide zigzag stitch works fine or long stitches to hold it together until it is quilted also works well. The stitches don't need to be close or neat, just enough to hold it together until it is quilted. You can also glue (spray glue or school glue) the backing and then just lay your batting pieces on making sure they butt together nicely and that will hold it all in place.