There are several ways... but I use something like this:
Every does it differently, but I go through various grits from 1500 to 0.5micron lapping discs. One can do it with a dremel I suppose. dissassemble the bezel and go slow with whatever technique you decide to use. Its very easy to round off the corners when polishing.. the edges should be as sharp/crisp as possible. I suggest you practice with a couple of cheap sacrificial cases first.