without having received/learnt any technique of meditation/Kriya Yoga i simply try to concentrate or meditate in the Agya Chakra (Dwidal Chakra) in 'aum' mantra. The time for such concentration depends on when i desire to perform this (during day or night time). No fixed time is maintained.
i would request you to enlighten me whether such practice is correct, or is it necessary to do it after proper training.
—Santanu Mukherjee, India
God bless you.
Both elements in your practice are good.
Concentrating on the spiritual eye (Agya Chakra) is something Yogananda taught and emphasized. In his Autobiography of a Yogi he writes:
“Long concentration on the liberating spiritual eye has enabled the yogi to destroy all delusions concerning matter and its gravitational weight; thenceforth he sees the universe as an essentially undifferentiated mass of light.”
Also the AUM mantra is a good practice: Aum is called the “mantra of mantras,” or the “King of mantras.”
Again from the Autobiography: “Aum, the blissful Comforter, is heard in meditation and reveals to the devotee the ultimate Truth.”
However, to be a bit more specific, Yogananda also taught sacred techniques (Hong Sau and Kriya Yoga).
In these techniques, first you concentrate on a specific object (on the breath with a mantra, or on the prana current in the spine) while keeping the gaze at the Ajna Chakra. Then, in the second part, you leave that technique behind and concentrate fully on the Agya Chakra, to experience a contact with God.
There is another reason for learning these techniques. They come with the powerful blessings of Yogananda, a great saint and avatar. He stated that through these techniques he will work with the devotees. This is an important statement, as the blessing of a true Guru is a major element for deep spirituality.
Lastly, since you say, “no fixed time is maintained,” Yogananda wrote: “Routinize your life. God created routine. The sun shines until dusk, and the stars shine until dawn.” He recommended to establish fixed times during the day for meditation, and to meditate always at the same time if possible. Best would be twice a day if you can.
May your meditations blossom into Self-realization,