Simplicity means that without diplomacy one should be so straightforward that he can disclose the real truth even to an enemy. As for acceptance of the spiritual master, that is essential, because without the instruction of a bona fide spiritual master, one cannot progress in the spiritual science. One should approach the spiritual master with all humility and offer him all services so that he will be pleased to bestow his blessings upon the disciple. Because a bona fide spiritual master is a representative of Kṛṣṇa, if he bestows any blessings upon his disciple, that will make the disciple immediately advanced without the disciple's following the regulated principles. Or, the regulated principles will be easier for one who has served the spiritual master without reservation.
Cleanliness is essential for making advancement in spiritual life. There are two kinds of cleanliness: external and internal. External cleanliness means taking a bath, but for internal cleanliness, one has to think of Kṛṣṇa always and chant Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/Hare Rāma. Hare Rāma, Rāma Rāma Hare Hare. This process cleans the accumulated dust of past karma from the mind.
Steadiness means that one should be very determined to make progress in spiritual life. Without such determination, one cannot make tangible progress. And self-control means that one should not accept anything which is detrimental to the path of spiritual progress. One should become accustomed to this and reject anything which is against the path of spiritual progress. This is real renunciation. The senses are so strong that they are always anxious to have sense gratification. One should not cater to these demands, which are not necessary. The senses should only be gratified to keep the body fit so that one can discharge his duty in advancing in spiritual life. The most important and uncontrollable sense is the tongue. If one can control the tongue, then there is every possibility of controlling the other senses. The function of the tongue is to taste and to vibrate. Therefore, by systematic regulation, the tongue should always be engaged in tasting the remnants of foodstuffs offered to Kṛṣṇa and chanting Hare Kṛṣṇa. As far as the eyes are concerned, they should not be allowed to see anything but the beautiful form of Kṛṣṇa. That will control the eyes. Similarly, the ears should be engaged in hearing about Kṛṣṇa and the nose in smelling the flowers offered to Kṛṣṇa. This is the process of devotional service, and it is understood here that Bhagavad-gītā is simply expounding the science of devotional service. Devotional service is the main and sole objective. Unintelligent commentators on the Gītā try to divert the mind of the reader to other subjects, but there is no other subject in Bhagavad-gītā but devotional service.