How to get from Paris EST train station to CDG airport?
From Paris Gare de l'Est, you can either walk to Gare du Nord or take Metro line 5 one station in direction Bobigny-Pablo Picasso until Gare du Nord station.
From Gare du Nord transfer to RER B train in direction Aeroport Charles de Gaulle 1 or 2.
You can purchase a single RER ticket at Gare de l'Est to use for the entire trip from Paris to CDG airport. You don't need a separate Metro ticket + RER ticket.