Time to Go....

Easiest? I'd take the RER B to Gare du Nord, then change to the RER E to St Lazare. The Thalys station is at Gare du Nord, so just go back the same way.

Have a great trip!!!

Thanks Sam, what is the RER?

The Réseau Express Régional colloquially called the RER, is a hybrid suburban commuter/rapid transit system serving Paris, France and its suburbs. The RER ...

And the routes are designated by letters. "B" happens to be the line running from CDG into Paris. Cost is 10 EUR from CDG to anywhere in Paris on the RER or Metro (the other rapid transit system whose lines are designated by numbers). So you get off at Gare du Nord and follow the signs to RER "E" and go to the platform designated direction St Lazare. At St Lazare, follow the signs to "Trains a Grand Ligne" (mainline trains). Hold on to that RER ticket, as you need it to exit the station.

Returning, the RER "E" ticket is only 1.90 EUR to Gare du Nord. Again, hold on to the ticket to exit the station.

I disagree that the RER is the easiest way. Easiest in my opinion is a taxi for 50€ (flat rate for up to 4 people and their luggage), especially for a person who is leaving tomorrow and does not know what the RER is.

A different perspective- we fly from the West Coast and are usually a little off our game when we land, so we take a taxi to our next destination. Once we recover from our jet lag, we use public transportation. It's all in what your comfortable with. I have such respect for people who are able to get off the plane and navigate a new area for the first time. French taxis charge a flat rate from the airport, so you don't have to worry about that.

Have a wonderful trip.

Thanks everyone! As usual, very helpful 👍😄

Judy,
I did much of your trip last year. I think I googled "Paris public transportation map" , and did the same for schedule (maybe Paris Bus schedule) and got a map with all lines of all types of transport (RER, metro, and bus), and could pick the one that looked most straightforward. I now can't recall what we took (!), but I don't remember it being difficult at ALL. You will have SO much fun. Amsterdam is so fun, the Bayeux tapestry is amazing, Mt. St. Michel was wonderful (we arrived at 5p and it was nearly empty of people, in June). Have a great trip!

Judy:
http://www.ratp.fr/itineraires/en/ratp/recherche-avancee
All about the capital's mass transit: Subway, suburban rail integrated with the subway (RER), surface buses and light rail (ie. streetcars.) Use the Getting Around section to plan your daily trips.
www.parisbytrain.com is a blog with photo tours of the main train stations. You can see how the RER station for St-Lazare requires a short walk on the street to get to the SNCF train station taking you to Normandy.

You still should get some kind of guide book e-book on France.
And read it on the long ride over.

Thanks Michael and Catherine! Very kind of you. I'm here safely after an eventful day.... two planes late with mechanical issues that caused me to miss the train I'd purchased a ticket for. Fare went from 15 to 37.60E 😳 Even with all the great help I got turned around in the RER system but kind people got me straightened out.

I've read the Rick Steves Normandy guide and plan on going in order as he does if I can resist the pull of the Cemetary to be first. Thanks again for all the help. Judy

You go Judy! If you can endure late planes, missed train and the Paris subway, and can keep your wits about you, you've got it made. Have a great adventure.
Thalys has their own section at the Gare du Nord. ( I can't remember the track numbers, but something like, 8,9,10 & 11) Remember that the trains often aren't listed until an hour or less before their departure, so don't be dismayed if you don't see your train. Thalys does have their own security, sort of like the airlines, only no zones, just one big line.