They are made of stainless so start with an aluminum oxide wheel on a grinder (it is a soft abrasive wheel that sorta feels like rubber) then switch to 3M Scotch Brite pads and then onto a polishing compound. The wheels are kinda pricey but last a long time. You can find them for a number of grinder types from bench to 4 1/2" disc. I generally use the Red or Brown 3M pads, they are finer grit. Any semi abrasive polishing compound should be able to achieve a high shine.
You could also do it with 3M wet or dry paper starting with 400 and progressing
on to finer grits.Use soapy water to wet it down and sand only in one direction.
If your not familiar with the process it's a lot harder to screw it up by hand,plus
it's a lot quieter and no need for for hearing or eye protection.