The closest metro stop is Volontaire on line 12 in the 15th area.
From the Charles de Gaulle airport:
Take a train called the RER B into the city of Paris (8.10 euros). Get off at the stop called Denfert-Rochereau and switch to metro line 6 (direction: Charles de Gaulle-Etoile) Get off at the stop called ‘Pasteur’, then switch to line no.12, direction Mairie D’Issy and get off at ‘Volontaires’ (1 stop).
As you go out turn right ( Rue des Volontaires) and on the first street you see turn left (Rue Blomet).after 3 blocks you’ll find the hostel, it’s on the corner of Rue Blomet and Rue Borrome.