There is a video on either YouTube or on the Flitz site itself that shows Diamond plate being polished. You can look up the video, but from memory, the idea is to wash the Diamond plate first, and then dry it. After it is completely dry, use the Flitz Pre Cleaner, allow to dwell for a short period of time, then wipe off. After that, put a thin layer of Flitz metal polish on the surface, and buff using the Flitz Buff Ball.
The Pre Cleaner removes any chemical contaminants and Aluminum oxides. The Buff Ball stands up to the "Diamonds" that are part of the Diamond plate. The Diamonds have sharp edges that will chew up a foam polishing ball. The Buff Ball is also washable (in a washing machine).