I gotta do the exact same thing with a Gottlieb decal I bought from Pinball rescue. Below are the instructions that came with my decal. Obviously mine doesn't have a signature on but perhaps if you do what the below instructions say, just a bit of detergent and water on the apron then put a piece of paper over the top of the decal to protect it (and the signature) whilst you work out any excess water and air.
