The easiest is via AIR FRANCE BUS , about 1 hour ( they have radios and circumnavigate traffic ) direct from any CDG Terminal to the back of the train station. .
By rail, you have first to take a (free) bus to the RER Station, take a RER card with Metro connection , take the RER train to City, exit at Gare du Nord, walk half a mile to get a Metro direct to Monparnassse, walk another half a mile to get to the train station departure hall.
Either way you are looking at 1 hour ( if road traffic and train connections are OK ) to one and half hour journey.