Here is how I would do it:

Get euros at a Cashpoint in CDG

Take the RER from CDG2 to Gare du Nord
Take Metro #5 toward(direction) Place d'Italie
Transfer(correspondence) to Metro #9 toward
Mairie de Montreuil at Republique or Oberkampf
Exit(Sortie) at Voltaire

Your RER ticket for 7.75 euros is also valid
for the Metro portion of this trip

Make sure Voltaire is your closest Metro stop
before following these directions