Getting to CDG from Square Caulaincourt:
Go to the LAMARCK CAULAINCAURT metro station.
Take the #12 (Green Line) in the direction of PORTE DE LA CHAPELLE.
Go two stops and get off at MARCADET POISSONNIERS. Don't get out of the station.
At that station, switch to the #4 (magenta) line in the direction of PORT D'ORLEANS.
At the third stop, GARE DU NORD, get off and make your way to the platform for RER B.
Take the RER B in the direction of AEROPORT CH. DE GAULLE and get off at the airport.
At the airport, find out which terminal EasyJet leaves from and, if necessary, take the shuttle to that terminal.
Getting from Luton airport to London proper:
The tube doesn't serve Luton airport. You'd have to get on a free Thameslink shuttle to the Thameslink rail station at Luton Airport Parkway. From there, you'd take the train to Kings Cross station, at which point you'd hop on the tube to get to wherever in London you're going.
Since you're taking EasyJet, you'll probably find it easiest to take EasyBus
for £2 (advanced booking) or £7 (at the bus) into Central London.