Landing at CDG at 9:40am Tuesday. What is the most efficient way to get on a train to Cherbourg?
Do you mean tomorrow, 10 February? If so, take the RER B from CDG to Gare du Nord. Walk through the connecting tunnels to the Magenta RER station. Take the RER E to Haussmann-St Lazare. Walk to Gare St. Lazare. The fare is €8.40. Go to www.parisbytrain.com for information on finding either of the two RER stations at CDG. Go here for a zoomable map of the Metro and RER system.
If your flight is on time, you might be able to catch the train leaving Gare St Lazare at 12:10 and arriving in Cherbourg at 15:09; but don't count on it. The next train leaves at 13:45, you'll have 8 minutes to change trains in Caen, and you'll arrive in Cherbourg at 17:11.
If the Tuesday in question is sometime farther in the future, then the timetables I gave you may not be accurate. When I entered an arbitrary date in May on both www.voyages-sncf.com and www.bahn.de, I found some changes.
It appears the only routing would be the RER into Paris Nord, a quick cab over to St. Lazare station, then from there to Cherbourg. You've got about 6 trains possible...some that require a change in Caen, and some that do not. The first would be the 12:10 train, #3307, leaving St. Lazare and getting into Cherbourg around 3:10 pm. Travel times are between 3 and 4 hours from St. Lazare, depending on the one chosen.