To use the RER train system to get you to charles de Gaulle airport you first have to use the airport's Orlyval shuttle train service from the airport to the RER station 'Antony' where you pick up the RER line B3.
The Orlyval is the shuttle service connecting Paris's Orly Airport with the city's RER transit network which you need to access for onward travel to Charles de Gaulle Airport (CDG).
There a regular service from Orly Airport to Antony RER station, from 6 a.m. to 11 p.m. every day of the week.
These automated, driver-less trains run all day and every four minutes during morning and evening peak periods.
This method of getting to CDG airport may not suit everyone as the trains can be busy at peak times and handling luggage and young children might prove difficult - although it can be a good option for budget travellers and those who don't mind hauling their luggage through turnstiles and up some escalators.
The Orlyval / 'Paris by train' signs are well situated and easy to follow.
You can buy your ticket for the journey at the Orlyval ticket booth within the airport then use the escalator/elevator up to the the elevated Orlyval train station.
The combined fare, OrlyVal/RER Orly to CDG airport is € 24.60 (as at Jan. 2023)
ORLYVAL runs every 5 to 7 minutes, from 06:00 to 23:35.
The ticket is valid for the Orlyval and RER train journey to Charles de Gaulle airport.
It is a short train journey ( 8 mins ) to 'Antony' station, you have to use your ticket to get through the barriers as you arrive. If you have bulky items there is normally an attendant on hand.
Once at Antony the RER trains are well sign-posted.
From the Antony RER station, you take the train B3 in the direction of Aéroport Charles de Gaulle' and not the ones marked to 'Mitry Claye'.