Its quite easy to do but takes a lot of time and sore fingers! Personally I would start with some medium grade sandpaper to start smoothing over any rough bits, use a file to clean up any sharp edges or daggy bits, cast lines etc. Then move to finer sandpaper until you get down to very fine wet/dry. Then get some Autosol or another brand of metal polish, a bunch of wrags and start polishing!!
I've polished a few cast alloy parts this way and they come up pretty well. The more effort and time you put into it the better the result.
