What is the correct way to attach a receptacle to a metal face plate?
What you have there is called an RS (for raised steel) cover, which attaches to a 1900 or 11-B box. Since the 1900 box provides no support for the wiring devices, you really should be attaching them using the screws and nuts that come with the RS cover, in addition to the center screw. Don't forget to ground the box as well, using a green ground screw. 

This opposes a similar raised cover called a mud ring, which provides the support that you would get from an ordinary rough in box. In that case, you'd be attaching a typical trim to the wiring device using just the center hole.