Arrive at terminal 1. Get lifts (ascendeurs) which are well hidden to floor 2 ("aeroshop") where you get the shuttlebus to the RER.
Go outside - opposite is the stop where you catch the green free number 2 bus to the other terminal (9). When you arrive, ticket office is on your left inside the building. "un billet aller-simple pour Paris, SVP" is sufficient - costs around 8E. Platforms to Paris are clearly marked in English, trains are frequent and get the stopping service to Chatelet les Halles or Saint Michel.

On the way back, get off at the first RER stop, catch the Ligne 2 (green) bus to terminal 1, go through the doors & up the escalators. Check in, then get food BEFORE going through passport control. Leave extra time before boarding since the x-ray machines & metal detectors are in the satellites just before you get on the plane and sometimes there is a big queue.