It sort of depends on what you consider easy, what tools you have available or are willing to buy and how many you need to make. A piece of sandpaper glued or taped to a flat surface is perfectly sufficient, but may not be the most accurate. A simple jig for holding and indexing can be used with anything be it a hand file, block plane, router, belt sander, disc sander, milling machine, lathe, really whatever you have handy that cuts. Shawn Newton has some videos on Youtube of him cutting large facets on his pens with a hand file and also a mill, I believe. If you look for videos on making chopsticks you'll find a number of small jigs used for faceting those.