- Take a taxi. That's an easy one, you just walk out of the terminal, get to the taxi queue, wait, enter a taxi, tell the driver "Bonjour, gare Montparnasse s'il vous plait", and some time later you will get there. There's a flat fare of 55 euros.
Pros: convenient. Cons: more expensive than other options. May take anywhere from 45 minutes to 2.5 hours depending on traffic. Unless you do that on a Sunday, given the time you land, it will probably take a while.
Use Uber or one of the equivalent services. May be cheaper, but not necessarily, and will take slightly longer as they are not allowed to use bus lanes.
Take the RER and metro. If you land at T2 (except T2G), walk to the train station in the terminal, buy a ticket to Paris (about 10 euros per person), board the RER, and change at Denfert-Rochereau to line 6, towards Charles-de-Gaulle Etoile (not the same as Aéroport Roissy Charles-de-Gaulle). Alight at Montparnasse Bienvenüe.
If you land at T1, you'll have to take the (free) people mover (CDGVAL) to Roissypôle where you'll take the RER. If you land at T3, you need to walk to Roissypôle. If you land at T2G, you need to take a shuttle bus to the train station.
There are usually direct (from CDG to Paris Gare du Nord) and omnibus trains alternating on the line. You probably want the direct one. Not much faster than the omnibus one, but less busy until Gare du Nord.
There are two metro lines from Denfert-Rochereau to Montparnasse-Bienvenüe. You do not want to take line 4, as Montparnasse-Bienvenüe station is very large, and line 4 gets you to the very opposite from the train station.
Pros: cheaper (especially if you are alone). Cons: not very convenient if you have lots of luggage. The RER will probably be quite busy at that time of day. Make sure you look after your belongings.
Note that during peak times, this is probably the fastest option (about 1 hour), though definitely not the most comfortable.
- Take Le Bus Direct which goes directly from CDG (3 different stops) to Montparnasse.
Pros: direct, convenient. More expensive than the RER option, but cheaper than taxi if there are fewer than 3 people travelling.