Commentary of Saint Ramanuja’s Bhagavad Gita Chapter 6 Verses 13 & 14:
The word saman means straight this denotes that the back, neck and body should be erect and balanced in a straight posture. By sitting with legs crossed in the lotus position or half lotus position assists in keeping this straight position. The word sthira means firm this denotes that the seat while being comfortable should not be overly soft. The eyes should not be allowed to flit hither and thither in different directions on objects near or far but should fix their focus either on the tip of the nose or on the space between the eyebrows. That was the discipline for the body now Lord Krishna gives the discipline for the mind. The mind being tranquil connotes contentment along with freedom from all fears. The compound word brahmacari-vrate means total celibacy and is an indispensable component insuring the containment and preservation of the vital energy of the physical body which is so essential for development. Keeping the mind attentive and fixed internally one should meditate on the Supreme Lord. Courtesy: www.bhagavad-gita.org
Summing Up
a. Neck, Back and Body should be straight and in balance
b. Focus should be in the tip of the nose or on the space between the eyebrows (Ajna Chakra in Kundalini meditation)
c. Mind fixed on Supreme Lord
My experience here is that “Start with your gaze on the tip of the nose or space between the eyebrows”.
Slowly you will able to fix your mind on Supreme Lord. There after you will get your posture correct automatically.
Caution: Practice always with Guidance..