There's an airport shuttle that stops at the various terminals at CDG, as well as TGV and RER station. See map here.

I typically arrive into Terminal 2A, board the shuttle which passes 2C before stopping at the Gare TGV (shared with RER IIRC). Tickets for RER are purchased at either the vending machines or you can queue up to buy from sales clerks. Coming back make sure you exit the correct terminal (1 or 2), as the RER stops at both.

HTH,
benzguy80