From Lourve/Rivoli metro to CDG Airport

Take Metro 1 east one stop to Chatelet and catch the RER B to the airport. You'll have to do some hiking between the two train platforms.

Yes, the first reply is correct. That's how you do it. BUT: I'm chiming in to add a word to the wise. (I'm hideously embarrassed, but I feel my pain might be someone else's gain. :o) Coming home from Paris last month, my husband and I took pretty much this exact route. We were unable to locate a ticket machine on the platform for the RER B, and since the correspondance didn't involve going out any gates (that we could tell) we just... didn't buy the RER tickets. It was an insanely stupid move, but it's an easy mistake to make. Some Metro stations appear to allow you to walk right down (or over) onto the RER B platform, without getting a ticket. Imagine our surprise when we disembarked at CDG and were met with a testy official demanding to see tickets. We pleaded ignorance. We pleaded poverty. We begged. We grovelled. We ended up paying 40 Euros EACH for the privilege of getting off the train. They charged us not just the ticket price, but a penalty. So be careful!

Argh! You need to buy the ticket for the whole trip the first time you go underground. If you don't, you need to go out the turnstile to the ticket place and buy the one for the rest of the trip. If you do it this way it costs mores, since you're essentially double-paying for the first part of the trip that you just finished. People make the same mistake going out to Versailles.

If your flight departs from Terminal 1, get off at the CDG 1 RER station which is next to Terminal 3 and take the free 24/7 CDGVAL driverless shuttle train to Terminal 1. If your flight departs from Terminal 2, stay on the train for one more stop and get off at the CDG 2 TGV RER station. For detailed information, including photos, go the the Paris by Train site.