To start with, you must have downloaded albums/playlists. To know how to do this please go to "How do I download an album or playlist on the Deezer app?".
To force offline mode go to settings on the app and click the" Offline Mode" box. Once this has been activated the Deezer app will not use mobile network and you will not be able to access the tracks on-line. Only albums and playlists which have been downloaded can be listened to.