Use wet 1000 grit sandpaper and light pressure to sand the plastic in circular patterns. When you notice that the small scratches look identical across the entire headlight, sand horizontally until all the small scratches appear horizontal.

Use a spray-bottle filled with water to keep the sandpaper and the headlight wet throughout the entire sanding process. Dry sanding can damage plastic.

Repeat the process with 1500, then 2000 grit sandpaper.