As others have mentioned, it's not all that difficult.
I asked for some advice here recently, regarding a Campagnolo seatpost that had some zig-zagging marks.
I ended up wet sanding,, starting with 500/then 1000/and 1200. Some Mothers mag polish and a soft towel to finish. I might have an hour in it.
If your stem is free from any scrapes/gouges, you can probably start with something finer than 500(6-800). I'll also note that using a vise helps a lot(lined with wood).
Before
After