[XP] Inheritance

[xuroth]

Hey everyone,
I have a question about inheritance. If a new class has inherited from a RTP (or standard) class, it gets all the methods, so I believe...
However, if I just want to add to that method, do I have to define it again and list only the additions? or define it and list the entire method + additions? or am I supposed to alias it and just add the additions? (basically, I know what alias is supposed to do, but no real clue how to use it yet.)

for example

Im such a noob at this...

Code:

class my_item_class < Scene_Item
#my code
   def main #(do I need to alias this instead? How should I do that?)
    # original code from Scene_Item's def main (do I need to include this?)
    #my added code (or do I just add this?)
   end
#sorry for the noobish question, but the help file wasnt really clear (or i may have missed something)
end