How do I buy a reserved train ticket from Paris (CDG) to Épernay using this 'World Wide Web'?

Do you speak French?

Hmm...Not a complete answer here, but a little surfing around the SNCF.Fr site turns up "Roissy (95)" as the Charles de Gaulle railway station. I'm looking at this http://www.voyages-sncf.com/dynamic/_SvTermCommVoySaisie?_TMS=1179421504429&_DLG=SvTermCommVoySaisie&_LANG=FR&_AGENCY=VSC</a%20

I believe the CDG-1 and CDG-2 are the terminals. http://www.shuttle-paris.com/roissy-cdg-airport-map.htm Just use the sncf.fr website to go from your terminal to Épernay, voila!

Also, they will speak english at the train station where you buy tickets, if you want to wait.

OK. try this http://www.voyages-sncf.com

http://www.parisdigest.com/transport/charlesdegaulle.htm might be useful. Looks like there's two ways to get the train - TGV and RER. RER is the local overground metro service, you probably can't book in advance, but it's likely to be much cheaper than the TGV, which is the superfast posh train which you probably can book in advance. http://en.wikipedia.org/wiki/Charles_de_Gaulle_International_Airport indicates that the TGV station is attached to Terminal 2, so you might be short of luck if you're arriving into Terminal 1. You're right that Charles de Gaulle-Etoile is not the station you want - it's a metro station in central Paris. You don't say how much you know of France, French language, French transport systems etc., so I'm assuming you know very little - sorry if this isn't the case.

All the SNCF website searches I have tried bring up results that take you on the RER, i.e. that aren't bookable in advance.

Yes, I've been trying that site (SNCF). When I enter "Charles De Gaulle 1", it suggests "ROISSY (95) - Station served: CHARLES DE GAULLE". When I actually try to book this, it says (now), " The departure city null you entered is unrecognized. Please check spelling and try again." Earlier, it was saying something to the effect of, "Your itinerary requires more than three transfers, so you can't book it online." I have it on good authority that there is a train that goes straight from Paris to Epernay, so more than three transfers seems a but excessive. Altolinguistic-- my French skills are on the basic traveller's level. My knowledge of ex-Parisian transit, which I'm beginning to doubt, is "short->Metro, medium->RER (thats the tall one), long->TGV." So originally, I was googling "book RER ticket". I just figured you could book those online. This is related to my confusion re: which is the correct departing station even, because if the TGV leaves from one but the RER leaves from the other, won't I only see half the schedule on the website? In short, I know just enough about French rail travel to cause problems for myself.

What you want actually involves 2 legs and 2 different systems of transportation: from CDG2 to Paris, then from Paris to Epernay. For the first leg, you have several options listed http://en.parisinfo.com/rub6203.html&id_article=6726, for example: airlines coaches, regular bus, RER (which is the Paris regional Metro, linked to the metro network). Specifically, you want to go to "Gare de l'Est", which is the Paris train station for lines going East. If you use the metro, there is a "Gare de l'Est" metro station. For the second leg, you go to the http://www.voyages-sncf.com/dynamic/_SvHomePage?_DLG=SvHomePage&_CMD=cmdHomepageUK&WB=HP and buy directly your ticket on line from Paris to Epernay. You can choose your language, select date and hours and obtain several options. Once in Épernay, enjoy champagne, lucky dog.