Did a few.. Nothing really to it.
Add pressure treated surface contact purlins to the underside of the decks floor joist.
Screw the roof material (steel, fiberglass) to the purlins.
Add gutters.

Yes it is a little more complicated than that with the cutting, trimming and working with beams. But that is the basics.

Hippy is right. If you have an already built deck, you can cover the bottom of the joists with a dry space system. It collects the water between the gaps.