Spray the surface of the plastic scuff guard down with water, and then use the 3,000-grit sandpaper without putting pressure on the surface to smooth out the scuff. This will remove the scuff as you work without damaging the plastic or scratching it even worse.
Dry the surface with the clean rag, and then squeeze a quarter-sized dollop of polishing compound directly onto the scuff.
Rub the polishing compound into the plastic with a dry spot on the clean rag. Doing this, you will begin to see the haze that was created by the sandpaper start to fade until it is gone completely.
Wipe away the excess polishing compound from the plastic scuff guard with another dry, soft cloth, buffing in circles until the polishing compound is gone.