Check this thread out and ask the poster any questions;
https://www.e90post.com/forums/showthread.php?t=1679614
Basically;
You'll smooth out any imperfections and scratches with sandpaper. Going from a rough grit gradually to a finer grit, like 600/800 up to 2000-3000 grit, then you'll use a polishing compound to get the mirror finish after the sanding, then seal them or clear coat them.