As above, flat dolly pushed hard into the dent from inside the door, then gently tapped with a polished planishing hammer from the front, dont over do it as the metal will start to stretch.. DONT do it with any type of grip or pliers, you will make the dent bigger, and damage more of the paint
Those door plates are a complete waste of time, someone has broken into your car, and only made a small dent and easily repaired, Ive repaired doors that have had the plates fitted, and the doors have been butchered beyond belief.. the first thing I did when I got my Golf was to get shot of them! the only real use they have is to hide expensive damage on a car that doesn't warrant a proper repair...
Tom