Kṛṣṇa tells Arjuna that he should act in yoga. And what is that yoga? Yoga means to concentrate the mind upon the Supreme by controlling the ever-disturbing senses. And who is the Supreme? The Supreme is the Lord. And because He Himself is telling Arjuna to fight, Arjuna has nothing to do with the results of the fight. Gain or victory are Kṛṣṇa's concern; Arjuna is simply advised to act according to the dictation of Kṛṣṇa. The following of Kṛṣṇa's dictation is real yoga, and this is practiced in the process called Kṛṣṇa consciousness. By Kṛṣṇa consciousness only can one give up the sense of proprietorship. One has to become the servant of Kṛṣṇa, or the servant of the servant of Kṛṣṇa (CC-2:13:80). That is the right way to discharge duty in Kṛṣṇa consciousness, which alone can help one to act in yoga.
(Bhagavad-Gita--2:48---purport).

In the pursuit of self-realization, one has to follow many rules and regulations to control the mind and the senses and to concentrate the mind on the self. All these procedures are very difficult, bitter like poison, but if one is successful in following the regulations and comes to the transcendental position, he begins to drink real nectar, and he enjoys life.
(Bhagavad-Gita---18:37---purport).

Samādhi involves concentrating the mind upon the supreme cause of all, even if one is unaware of whether His actual nature is personal, impersonal or localized. Concentration of the mind on the Supreme is certainly a form of devotional service. To cease from personal sense endeavors and to concentrate on the supreme cause is a sign of self-surrender, and when self-surrender is present, that is a sure sign of devotional service. Each and every living entity needs to engage in devotional service to the Lord if he wishes to understand the ultimate cause of his existence.
(Srimad Bhagavatam---3:8:21---purport).

When one is detached from the attraction of material prosperity, one can actually concentrate his mind upon the Supersoul. As long as the mind is distracted towards the material, there is no possibility of concentrating one's mind and intelligence upon the Supreme Personality of Godhead or His partial representation, Supersoul. In other words, one cannot concentrate one's mind and energy upon the Supreme unless one is detached from the material world. Following detachment from the material world, one can actually attain transcendental knowledge of the Absolute Truth.
(Srimad Bhagavatam---3:26:72---purport).

The success of one's life is measured by one's material happiness in this life and liberation in the next. Such success can be achieved only by the grace of the Supreme Personality of Godhead. The words eka-matyā mean concentrating one's mind on the Lord without deviation.
(Srimad Bhagavatam---4:8:21---purport).

According to different relationships with the Supreme Personality of Godhead, there are varieties of nāmānukīrtanam, chanting of the holy name, and thus according to different relationships and mellows there are five kinds of remembering. These are as follows: (a) conducting research into the worship of a particular form of the Lord, (b)concentrating the mind on one subject and withdrawing the mind's activities of thinking, feeling and willing from all other subjects, (c) concentrating upon a particular form of the Lord (this is called meditation), (d) concentrating one's mind continuously on the form of the Lord (this is called dhruvānusmṛti, or perfect meditation), and (e) awakening a likening for concentration upon a particular form (this is called samādhi, or trance). Mental concentration upon particular pastimes of the Lord in particular circumstances is also called remembrance. Therefore samādhi, trance, can be possible in five different ways in terms of one's relationship. Specifically, the trance of devotees on the stage of neutrality is called mental concentration.
(Srimad Bhagavatam---7:5:23-24---purport).

This is not a very difficult process. However, one must learn it from an experienced person. Tad vijñānārthaṁ sa gurum evābhigacchet: (MU 1.2.12) one must approach a person who is already in the practice. The mind is always flying to this and that, but one must practice concentrating the mind always on the form of the Supreme Lord, Śrī Kṛṣṇa, or on the sound of His name. The mind is naturally restless, going hither and thither, but it can rest in the sound vibration of Kṛṣṇa. One must thus meditate on paramaṁ puruṣam, the Supreme Personality of Godhead in the spiritual kingdom, the spiritual sky, and thus attain Him. The ways and the means for ultimate realization, ultimate attainment, are stated in the Bhagavad-gītā, and the doors of this knowledge are open for everyone. No one is barred out. All classes of men can approach Lord Kṛṣṇa by thinking of Him, for hearing and thinking of Him is possible for everyone.
(Bhagavad-Gita--introduction).