Did you want to try this with public transit or are you OK with a taxi?
Option 1: Taxi from the taxi rank at CDG to Gare Montparnasse. Train from Montparnasse to Chartres. No need to buy this ticket ahead of time but I would download the SNCF Connect app so you can purchase your tickets when you get there. To me, this is easier than the machines.
Option 2: Take the RER B from CDG to Denfert Rochereau station. Change to either Metro line 4 or 6 to Gare Montparnasse. I have not done this change but I would expect there to be steps.
The taxi is a fixed rate from CDG in to Paris. For the Left Bank, where Gare Montparnasse is located the rate is 65E. I usually hold up my CC while the driver is loading my bag and ask OK? I also hand them a 3x5 index card with the name of my destination on it and 65E written out beside so they know I know they know...blah blah, lol. No tip needed but if you do, just 2 or 3 E. You can say to the driver to add 2 or 3 to the total fare.
https://easycdg.com/taxi-shuttle-paris-cdg-airport-charlesdegaulle/