Lots of ways of doing it. My favorite way is to coarsely crack good coffee beans, try 1/2 lb to start, and put them into a fine mesh bag, into the keg and then pull the bag when the flavor gets just where I want it. No guesswork.
EDIT - I like to use Sumatra , French roast, or espresso beans best normally, depending on what I'm looking for. Fun to experiment though.