Hi Mike how much pickguard material do you need to remove ? There are holes for the pup mount screws so hopefully if it has to be made wider these won't be an issue. As Pest and DK have said use a file to rough out the shape and then sandpaper to get the edge nice and smooth. Good way to mark out the new shape use tape to mark the boundaries of the P90.
As Pest says clamp it down securely so the pickguard doesn't snap in half ! Minimise overhang from the bench so it can't move much. Or place it in a vice with cardboard to protect the surface