I regularly go to France via AC. When you clear customs and pickup your luggage and exit to the main terminal, make a right and walk all the way to the end of the terminal A. Then the pathway will be narrower and you will see a mcDonald's (free wifi if needed) and continue walking.. Then you will see an escalator for car rentals, a pharmacy and a small grocery store downstairs, keep walking... You will then enter CDG 2C, again, walk right past through that terminal as well. Then you will see see moving escalators and at the end (on your left), you will see a Sheraton as well as the SNCF and RER station below. It is called "Paris by train". If needed, the CDG Val is on the right which takes you to T1 or T3. There is another RER station at T3.

Oh and grab a croissant from Paul booth, to get the taste out of the terrible AC breakfast :-)

Edit: I just realized that you are coming from LH, so ignore my directions. Take the CDGVal and get out at T3/roissypole and the train station is located in the middle. There's a brioche doree where you can grab a croissant here as well. :-)

Have fun.