I would have really appreciated having these instructions so I am passing this along....
For the Roissy Bus go to Terminal 1, Level 3. You won't see any signs for it until you are that close. There is a small ticket office outside. You will cross one lane to get to it. Rick's book said they are only at Terminal 1 but the CDG website says they are located at all three terminals. Try going to the top floor of your terminal to find it if you do not come into Terminal 1.
The bus will drop you at the Opera. For our hotel on Rue Cler, we were to take the metro from the Opera metro stop but we never did find it and two of the locals we asked didn't know where it was either. Ask the bus driver to point you in the right direction. It just was not apparent to us and we walked around the Opera twice. From looking at Google maps, it looks like it is just SW of the opera.
Hope this helps. Good luck!