I have taken several different routes from CDG to Paris and I can only truly recommend the RER B train. It’s cheap, fast, and easy. It’s about a 30 minute trip as opposed to a van, bus, cab or limo.
There are signs guiding you to the RER inside the airport and if lost, ask around. If you are only going into Paris for a long layover, there is a company called BAGS OF THE WORLD ( Baggage du le monde) and you can check your bag for about $8 a day.
Attached is a video of hour to purchase your tickets and take the train. Study the Metro Guide and you can get anywhere via the metro when you get into Paris.