It's been covered before. Icebike.org I think has a nice walk-through on "winterizing your freehub".
Think Kimmo has posted about it too.
You do need a kinda-sorta special tool to unscrew the bearing cup in the freehub body. I've used a file that fitted in the notches. Some have managed with a suitable piece of sheet metal. One guy ground down a cone wrench. Don't forget it's a left hand thread.