as a real easy start, look to see if there as a drain plug on the bottom of the radiator (like a stopcock).
if so, then open it and let all the shit drain out. (preferably onto grass??).
stewart - id be rather wary of simply undoing the drain plug on old rusty cooling systems - the number of times ive seen the things never make a full seal again when you tighten them... if its not leaking, my moto is to not open it
i prefer to just drop the bottom hose off - much faster too
and regarding the refilling process - note that it may take SEVERAL attempts at filling the system before you can be assured that there is no air left. take it for a drive, turn the heater on, park it on a slope etc etc ... and just keep adding water over the next few days