At the airport, follow the Baggage-Exit signage then indications for Trains RER B.
Take the RER Line B to Châtelet-les-Halles (all trains call at Châtelet-Les Halles).
At Châtelet-Les Halles, change for the Metro line 14 (Direction Olympiades) to Bercy.
At Bercy, walk for approximately 350 meters towards Gare de Bercy.
At Gare de Bercy, walk for approximately 350 meters towards Bercy Metro station.
At Bercy, take the Metro Line 14 (Direction Mairie de Saint-Ouen) until Châtelet-les-Halles.
At Châtelet-Les Halles, change for the RER Line B (Direction Aéroport Charles de Gaulle).
Depending on your terminal, get off at station Aéroport Ch. de Gaulle 1 or Aéroport Ch. de Gaulle 2.
At the airport, proceed to your departure terminal.