I wouldn't flush the hub unless you are having issues. Just add a few drops and run it. Flushing doesn't get out any real goop if it is an issue and only serves to eradicate the bearing grease that keeps the oil from oozing out past it and thereafter the hub will constantly dribble down the spokes onto your rim.

If you just want to take it apart and play or there is a shifting problem that isn't just the cable or the trigger then this is the video for you.