The Astanga-yoga process is outlined very specifically in the Sixth Chapter of Bhagavad-gita :
“One should hold one’s body, neck, and head erect in a straight line and stare steadily at the tip of the nose. Thus, with an unagitated, subdued mind, devoid of fear, completely free from sex life, one should meditate upon Me within the heart and make Me the ultimate goal of life.” Yoga does not mean going to some class, paying some money, engaging in gymnastics, and then returning home to drink, smoke, and engage in sex. First of all, one has to go alone to a holy place and sit in a straight line, holding one’s body, neck, and head erect, and stare steadily at the tip of the nose. Why is this? This is a method to help concentrate one’s mind. That’s all. The real purpose of yoga, however, is to keep oneself always aware of God within.
One of the dangers of sitting in meditation and staring at the tip of one’s nose is that one will fall asleep. We would have seen many so-called meditators sitting like this and snoring. As soon as one closes his eyes, it is natural to feel sleepy; therefore it is recommended that the eyes are half closed. Thus it is said that one should look at the tip of his nose. With one’s sight thus concentrated, the mind should be subdued and unagitated. In India, the yogi often goes to a jungle to practice such meditation in solitude. But in a jungle, the yogi may think, “Maybe some tiger or snake is coming. What is that noise?” In this way, his mind may be agitated; therefore it is especially stated that the yogi must be “devoid of fear.” A deerskin is especially recommended as a yoga-asana, because it contains a chemical property that repels snakes; thus the yogi will not be disturbed by serpents. Whatever the case—serpents, tigers, or lions—one can be truly fearless only when he is established in Kṛṣṇa consciousness. Due to perverted memory, the conditioned soul is naturally fearful.The yogi must also be “completely free from sex life.” If one indulges in sex, he cannot concentrate; therefore brahmacarya, complete celibacy, is recommended to make the mind steady. By practicing celibacy, one cultivates determination.
The perfection of yoga means keeping the mind in a state of equilibrium. Materially speaking, this is impossible. After reading a mundane novel once, we will not want to read it again, but one can read Bhagavad-gita four times a day and still not tire of it. One may chant someone’s name a half an hour, or sing a mundane song three or four times, but before long this becomes tiresome. However, can be chant God’s name day and night, and one will never tire of it. Therefore it is only through transcendental vibration that the mind can be kept in a state of equilibrium. When one’s mental activities are thus stabilized, one is said to have attained yoga.
Ref >> Srila Prabhupada Vani.