See the map on the homepage to orient yourself to where the apartment is located. It's across the Seine from and a little East of the Eiffel Tower. The nearest metro station is Iena, on the number 9 line, one stop east of Trocadero. The address is 6, rue de Longchamp, 75116 Paris.
From Charles De Gaulle/Orly-
The RER/Metro is by far the easiest and most cost effective way to get into the city.
From DeGaulle- Take the RER B (It's the only RER line serving the airports) to the city. At Chatelet-Les Halles change to the Metro Line 1 towards La Defense. At Franklin D Roosevelt change to Line 9 towards Pont de Sevres. Iena is the second stop.
From Orly- Take the RER B towards the city. At Denfert Rochereau change to the Metro 6 towards Etoile. At Trocadero change to Metro Line 9 towards Marie de Monteuil. Iena is the first stop.
The Air France Bus-
The Air France Bus runs from both DeGaulle and Orly airports to the Arc de Triomphe (Charles de Gaule Etoile metro stop) and Palais de Congress/Porte Maillot. It costs roughly €13 and leaves frequently. Get off at the Arc de Triomphe and look for signs to the metro...underground. Board Line 6 towards Nation and go three stops to Trocadero. You can exit there and walk less than 10 minutes to the apartment, or switch to the 9 train towards Mairie de Montreuil, go one stop to Iena just down the street from the apartment.
Taxi- Yes you can take a taxi from either airport but it is quite expensive!!