Most information is already in the thread dawntreader linked. My top nut could be turned by hand when the weight of the rudder was taken off it (wedge under the blade), but if it won't, a makeshift tool would be two screwdrivers in the opposite holes and then a wrench across the top, between them to turn.
Do support the rudder from below, and make sure there are two people to hold on to it and let it down gently, the rudder post is solid steel and very heavy.