I strongly recommend the RER train. Follow the signs when you clear immigration/customs, it should be on your right hand side. It is quite a walk to cross terminal 2 to the train station, especially when crowded and if you have heavy luggage. When you get to the train station, get you ticket from a booth, it's around 9 euros. When you cross the gate with your ticket, pass your luggage FIRST in front of you. The automatic closing doors are really quick to close and I ruined one or two piece of luggage in the past... Also, keep you ticket with you, you'll need it to get out of the boarding area at Gare du Nord, or Chatelet. Hope that helps!