I just read the excellent Photo tutorial on taking the train from CDG into Paris, What seems to be missing is how to find your stop (in my case Notre Dame) and then transfer from RER B to RER C so I can get off at Eiffel?
On board the RER train, above every second door (sometimes each door) is a panel with stop names and yellow lights beside each name. The next stop will be flashing. Stops coming up will be fully lit. Stops passed will be unlit.
After you get off to make a transfer, there will be plenty of signage showing you connections you can make, such as to the RER C line to get to Eiffel Tower.