Your best bet using Paris transportation system :
Reach the station located in terminal 2.
Buy your ticket to Paris at the green machine if you have coins or a chip-and-pin credit card. Else you have to queue at the "regional" ticket booth, not the "grandes lignes" which is for long distance high-speed trains.
A ticket to Paris is good for any destination inside Paris, including metro lines.
Take the RER B at the dedicated platforms. Although there are several branches and express/local services, all trains serve all stops inside Paris. Therefore you can board the first one available.
The train stops at several stations inside Paris. Get off at Denfert-Rochereau.
Using the same ticket, transfer to the metro line 6. Follow the "Etoile" direction (westbound). Get off at Montparnasse-Bienvenue 3 stops later.
Follow the "Grandes lignes" directions to reach the Montparnasse train station. After you pass through the exit gates, take the sideways escalators up to get to the long-distance lines; or those in front of you to reach the commuter platforms.
Do not use line 4 for this trip, if possible. Line 4 serves Montparnasse and Denfert-Rochereau too, but its platforms are at the far end of the metro station; it would require you passing through several flights of stairs and the locally famous moving walkways for 6 minutes of walk at least and stays rather impractical when carrying luggage. In contrast, line 6 platforms are the closest to the train station.
Sometimes, when getting off the last evening TGV, you can see people running really fast while hauling their luggage through the metro station although it is 1am, since they hope to get the last line 4 train. By the time they reach the platforms, it is common that the last train just went off!