Because the Lord felt inconvenienced in crowded places, He asked Rūpa Gosvāmī to accompany Him to a place on the banks of the Ganges known as Daśāśvamedha-ghāṭa. For ten days He instructed Rūpa Gosvāmī about the truth of Kṛṣṇa, the principles of devotional service and the transcendental relationships with Kṛṣṇa. All of this was described in full detail so that in the future Rūpa Gosvāmī could distribute this science of Kṛṣṇa in his book Bhakti-rasāmṛta-sindhu. Indeed, Śrīla Rūpa Gosvāmī described this incident in the first verse of Bhakti-rasāmṛta-sindhu, in which he speaks of the causeless mercy of the Lord upon him.
(Teachings of Lord Caitanya).

Because these things are unknown to the conditioned soul, Lord Caitanya, out of His causeless mercy, instructed Rūpa Gosvāmī in the principles of devotional service. Later, Rūpa Gosvāmī distributed this science to the people in general.
In the prologue to the Bhakti-rasāmṛta-sindhu (1.1.2), Rūpa Gosvāmī describes Lord Caitanya as follows:

hṛdi yasya preraṇayā pravartito ’haṁ varāka-rūpo ’pi
tasya hareḥ pada-kamalaṁ vande caitanya-devasya
"I offer my respectful obeisances unto the lotus feet of Lord Caitanyadeva, the Supreme Personality of Godhead, because He has inspired me with the desire in my heart to write something about devotional service. For this reason I am writing this book on the science of devotion, known as the Bhakti-rasāmṛta-sindhu."

(Teachings of Lord Caitanya).

This is explained in the Bhagavad-gītā (7.19): "After many, many births of cultivating knowledge, one who has attained real knowledge surrenders unto Vāsudeva (Kṛṣṇa) because he can understand that Kṛṣṇa is present in everything, that He is distributed all over the cosmic manifestation." Although everything is under the control of the Supreme Lord and is situated in His energy, everything is nonetheless different from Kṛṣṇa in His personal form. It is stated in the Bhakti-rasāmṛta-sindhu that the platform of śama, on which one understands that Kṛṣṇa is the basic principle of everything, is the same as śānta-rati. Unless one is elevated to the platform of śānta-rati, one cannot be fixed in knowledge of the greatness of Kṛṣṇa or of the diffusion of His different energies, which are the cause of all manifestations.

(Teachings of Lord Caitanya).

The Lord instructed Śrīla Rūpa Gosvāmī to write the transcendental literature named Bhakti-rasāmṛta-sindhu, the science of devotional service, and indicate therein the substance of the five transcendental relationships with the Lord—śānta-, dāsya-, sakhya-, vātsalya- and madhura-rasa. It is explained in that great literature how the transcendental relationship of śānta-rasa, taking the shape of unflinching faith in Kṛṣṇa, is further developed into dāsya-rasa with the spirit of service, and then to sakhya-rasa, or undeterred fraternity, and further to the transcendental platform of parental love, wherein one feels himself to be maintaining the Lord. All these transcendental relationships culminate on the highest platform, conjugal love (madhura-rasa), wherein they all exist simultaneously.

(Teachings of Lord Caitanya).

The first ten of the twenty items mentioned above are affirmative, and the second ten are prohibitive. In the Bhakti-rasāmṛta-sindhu, compiled by Śrīla Rūpa Gosvāmī, it is said that one should be very liberal in behavior and should avoid any undesirable activities. Of the twenty regulations, the most important are the first three: to accept the shelter of a bona fide spiritual master, be initiated by him, and serve him.

(Teachings of Lord Caitanya).

To these thirty-five items are added another four: (1) marking one's body with sandalwood pulp to show that one is a Vaiṣṇava, (2) painting one's body with the holy names of the Lord, (3) covering one's body with the remnants of the Deity's coverings, (4) sipping caraṇāmṛta, the water that has washed the Deity. These four additional items make thirty-nine items for devotional service in all, and out of all of these the following five are most important: (1) to associate with devotees, (2) to chant the holy name of the Lord, (3) to hear Śrīmad-Bhāgavatam, (4) to live in a holy place such as Mathurā or Vṛndāvana, and (5) to serve the Deity with great devotion. These items are especially mentioned by Rūpa Gosvāmī in his book Bhakti-rasāmṛta-sindhu. The thirty-nine items above, plus these five items, total forty-four items. Add to these the twenty preliminary items and there are a total of sixty-four items for conducting devotional service. The devotional service of one who adopts these sixty-four items with his body, mind and senses gradually becomes pure. Some of the items are completely distinct from others, some are identical, and others appear to be mixed.

(Teachings of Lord Caitanya).

The inhabitants of Vṛndāvana, Vrajabhūmi, are living examples of devotional service. Theirs is ideal devotional service with attachment, and such devotion cannot be found anywhere except Vṛndāvana. Developing devotional service and attachment by following in the footsteps of the Vrajavāsīs is called rāga-mārga-bhakti, or devotional service in pursuance of attachment to the Lord. According to the Bhakti-rasāmṛta-sindhu (1.2.272), "The ecstatic attachment for the Lord experienced in the course of the devotional service that is natural for the devotee is called rāga, or transcendental attachment. The devotional service discharged with such deep attachment, and with consequent deep absorption in the object of love, is called rāgātmikā." Examples of such devotional service can be seen in the activities of the residents of Vrajabhūmi. One who becomes attracted to Kṛṣṇa by hearing of such attachment is certainly very fortunate. When one becomes deeply affected by the devotion of the residents of Vrajabhūmi and tries to follow in their footsteps, he does not care for the restrictions or regulations of the revealed scriptures. This is characteristic of one discharging rāga-bhakti.

(Teachings of Lord Caitanya).

All the inhabitants of Vrajabhūmi are very dear to Kṛṣṇa. A devotee selects one of them and follows in his footsteps in order to be successful in his own devotional service. A pure devotee discharging devotional service with attachment always follows in the footsteps of a personality of Vrajabhūmi. It is advised in the Bhakti-rasāmṛta-sindhu (1.2.294) that such a pure devotee should always remember the activities of a particular inhabitant of Vraja, even though he is not able to live in Vraja. In this way he can always think of Vraja.
(Teachings of Lord Caitanya).

"The word mat-parā is used only to refer to persons who are satisfied with the idea of becoming My adherents alone. They consider that I am their soul, I am their friend, I am their son, I am their master, I am their well-wisher, I am their God, and I am their supreme goal. My dear mother, time does not act on such devotees." In the Bhakti-rasāmṛta-sindhu (1.2.308), Rūpa Gosvāmī offers his respectful obeisances to those who always think of Kṛṣṇa as son, well-wisher, brother, father, friend, and so on. Whoever adheres to the principles of devotional service with attachment, following in the footsteps of a particular devotee of Vrajabhūmi, certainly attains the highest perfection of love of Godhead in that spirit.
(Teachings of Lord Caitanya).

In the Bhakti-rasāmṛta-sindhu (1.3.1), love of Godhead is compared to sunshine, and this shining makes the devotee's heart more and more lovely. The heart of such a devotee is situated in a transcendental position, beyond even the material mode of goodness. As the devotee's heart becomes increasingly sterilized by the sunshine of love, he attains a state called bhāva. This is the description of bhāva given by Rūpa Gosvāmī. Bhāva is the permanent characteristic of the living entity, and the crucial point of progress for bhāva is called the marginal state of love of Godhead. When the bhāva state becomes deeper and deeper, learned devotees call it love of Godhead.
(Teachings of Lord Caitanya).

This process is summarized by Rūpa Gosvāmī in the Bhakti-rasāmṛta-sindhu (1.4.15–16): "The first thing required is faith. Due to faith a person associates with pure devotees, and by such association he develops devotional service. As devotional service develops, his misgivings diminish. Then he is situated in firm conviction, and from that conviction he develops a taste for devotional service and advances to the stage of attachment for Kṛṣṇa, whereby he follows the regulative principles of devotional service spontaneously. After that point he makes still further progress and attains the state called bhāva, which is permanent. Such love of God becomes deeper and deeper, until it reaches the highest stage of love of Godhead."
(Teachings of Lord Caitanya).

"O Govinda! This young girl with tears in Her eyes is crying in a sweet voice, chanting Your glories." Such pure devotees are always anxious to describe the glories of Kṛṣṇa and to live in a place where He exhibited His pastimes. A similar verse appears, again, in the Kṛṣṇa-karṇāmṛta (92): "The body of Kṛṣṇa is so nice, and His face is so beautiful. Everything about Him is simply sweet and fragrant." And in the Bhakti-rasāmṛta-sindhu (1.2.156): "O lotus-eyed one, when will I be able to always chant Your holy name, and being inspired by that chanting, when will I be able to dance on the banks of the Yamunā?"
In this way Lord Caitanya described to Sanātana Gosvāmī the symptoms of the bhāva stage of devotional service.

(Teachings of Lord Caitanya).

Lord Caitanya next described the symptoms of actual love for Kṛṣṇa. He said that no one can understand the words, activities or symptoms of a person who has developed love of Kṛṣṇa. Even if one is very learned, it is very difficult to understand a pure devotee in the stage of love of God. This is confirmed in the Bhakti-rasāmṛta-sindhu.
(Teachings of Lord Caitanya).

The symptoms of highly developed devotional service for Kṛṣṇa, which are exhibited by the pure devotees, are sometimes imitated by those who are not actually pure devotees. This is described in the Bhakti-rasāmṛta-sindhu. Without devotional service to Kṛṣṇa, such symptoms are artificial, not actual. Sometimes those who are not conversant with the science of devotional service are captivated by the exhibition of such symptoms, but learned devotees know that they are simply imitation.
(Teachings of Lord Caitanya).