The train station is within the airport grounds, on the lower level, close to 2D, and signs are numerous. In the very unlikely event you'll get lost, you'll find airport staff (wearing distinguishable outfits) along your way and they'll be happy to guide you.
Tickets booth are available in the station. Beware of the vending machines, I'm not sure you can actually buy tickets to Chessy Marne-la-Vallée from there. Another option would be to buy the ticket online, you can do that on the SNCF website (
https://oui.sncf) and third-party vendors also sells them (Trainline comes to mind, their interface and customer service is one of the best).
(One gotcha: make sure you buy a ticket to
Chessy Marne-la-Vallée and not
Chessy. Chessy is a lovely city in Southern France. Easy to spot, the journey should be 10 minutes and not 5 hours.)
Train platforms are known usually 20 minutes before departure, and are displayed all around the train station. They also make announcements. The train number is shown on the screen before you go to the platform, on the platform itself, and on the train.
On the platform, a display will tell you approximatively where to go depending on your coach, for example you are coach 5 and need to go to mark “W”, just find the big “W” mark on the platform and you'll be good. If you don't have a seat assigned your ticket will not show you a coach, just go in any coach of your service class (1st or 2nd). This sounds like it's complicated but it's not, the train stays for a few minutes (so you'll have time) and if in doubt SNCF staff is available right on the platform to guide you (usually they step down on coaches 3/13).
Once on board, there is a lot of dedicated space for your luggage (between and inside the coaches). Since the journey is relatively short we usually use the jumpseats and stay close to the doors, but if it is your liking you can also take a seat (if you have one assigned, it will show up on your ticket, if not, use any available seat). Enjoy your high-speed ride at 300km/h to
Disneyland Paris!
One last hint: at the airport, if some young ladies come to you and insist you sign a petition or give some cash for a foundation, don't stop and go away, they are very talented pickpockets.