Yoga is union with the Divine, sadhana is what you do in order to unite with the Divine. You have to get away from the ordinary human consciousness and get into touch with the divine Consciousness.
For that call always on the Mother, open yourself to her, aspire and pray for her Force to work in you so as to make you fit—reject desire, restlessness, disturbances of the mind and vital. Dhyana means to make the mind and vital quiet and concentrate in aspiration for the Mother’s Peace, the Mother’s Presence, her Light, Force and Ananda.
Ref: The Mother with letters on the Mother