How do I get from Charles De Gaulle Airport to the Gare Montparnasse? What are the specific connections?
Walk out of terminal. Look for AirFrance bus Line 4. Get on. Don't get off at Gare de Lyon. Get off at Montparnasse.
Optionally, take the RER B to Saint-Michael Notre-Dame and change to Metro 4 for the rest of the way. There's some tunnel hiking and luggage lugging involved.
You can map out your journey at the regional transit authority's website http://www.ratp.fr/en/
Its estimate of the time required is probably a little optimistic.
This blog has photos of many of the train stations, which can be reassuring www.parisbytrain.com
Follow Ed's advice, much more pleasant way to go.