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 Mairie de Saint-Ouen) until Gare Saint Lazare.
At Gare Saint Lazare, take the Metro Line 14 (Direction Olympiades) 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.