This is still fresh on my mind because my fingers are just now about healed from polishing a set of slots. There is professional polishers that have fancy equipment and they probably do it a lot faster, but with diligence, anyone can do it.
The wheels I started with were '70s vintage Western 8.5" slots. They were in really nice shape with no curb rash or scratches. Someone had painted the slot openings black so the first step was to plastic media blast off the black so the cast would show.
I started with 320 and wet sanded until all the machine lines in the wheels were gone. I then progressed with 400, 600, 1000, 1500, and 2,000 grit paper. The wheels were actually beginning to shine with the 2,000 grit paper and the machine polisher with Flitz polish finished the job. The most time is with the first sanding to remove the machine lines and get the surface really smooth. It took me much longer than some of the others posted. I finally timed it on the last two wheels. it took me 6 to 7 hours per wheel!
The first picture is the clean wheel and the second picture in the far right is a wheel that is finished sanding, but not polished. The left wheel in the second picture is the finished product!