This section of the Hari-bhakti-vilasa 14.418-441,(the glories of Nrsimha-caturdasi) quoting from Brhan Nrsimha Purana, describes who Prahlada Maharaja was in his previous life and how he became a Narasimha Bhakta.
Prahlada said: O Supreme Lord! I bow down to You. O Lord Nrsimhadeva, I offer my obeisances to You. I am Your devotee. O Lord of lords, I now express to You my heartfelt inquiry. O Lord, how did I develop such intense devotion for You? O my Lord, kindly tell me-how did I become Your exalted devotee?
Lord Narasimhadeva said: O greatly learned one! O my son, listen carefully as I explain to you the reason for your devotion to Me that has made you very dear to Me.
In the ancient times, you were born as a brahmana but you did not study any of the scriptures. Your name was Vasudeva and you became attached to a prostitute.
In that birth, you became so attached to a prostitute that you did not bother to perform any pious activity, except for a single vow in relation to Me. As a result of observing that vow, you developed exalted devotion for Me.
Prahlada then said: O Lord Nrsimhadeva! O Acyuta! O Supreme Lord, who was my father in that lifetime? What was my duty? How did I observe Your vow, even though I was attached to a prostitute? Kindly explain all this to me in detail.
Lord Nrsimhadeva said: In ancient times, there was a brahmana who was well-versed in the Vedic literature. He lived in the city of Avantipura and was known as Vasu Sarma.
This pious brahmana used to perform fire sacrifice every day. He was very expert in performing all kinds of Vedic rituals and as such, he would satisfy the best of demigods by performing the agnistoma sacrifice. In that state of living, he did not commit any sinful activity.
His wife was very pious and humble. She was famous throughout the three worlds. She was chaste, expert in performing all kinds of pious activities, and always eager to serve her husband.
That exalted brahmana begot five sons within the womb of his wife. These sons were very pious, learned, and devoted to their father. Among them, you were the youngest and you became attached to a prostitute.
Due to being attached to a prostitute, you became accustomed to drinking wine and performing other sinful activities, so that you never engaged in the study of the Vedas.
You spent most of your time at the house of the prostitute. One day, however, you and her quarreled.
Because of that quarrel, you missed your meal that day and as a result, you unknowingly observed a sacred vow in relation to Me.
Because of that quarrel with the prostitute, you did not sleep the entire night. The prostitute also did not eat anything, nor did she sleep that night in your association.
Because of unknowingly observing this vow in relation to Me, throughout the entire day and night, both of you became purified and accumulated heaps of pious merit.
On the strength of observing this vow, the demigods are able to enjoy their lives in the heavenly planets. In order to acquire the power to create, Brahma had observed this vow.
Being empowered due to observing this vow in relation to Me, Brahma was able to create all moving and non-moving living entities. Mahadeva also observed this vow, so that he could kill the great demon, Tripurasura. It is a fact that by the strength of this vow, Siva was able to kill that formidable demon.
Many other demigods, ancient sages, and learned and pious kings, had previously followed this great vow in relation to Me.
By the influence of this vow, all of them attained the perfection of life. The prostitute also became devoted to Me and thereafter enjoyed great happiness within the three worlds. My dear child, it is for this reason that this vow in relation to Me is celebrated through the three worlds.
This vow came into existence just to deliver such rascals. O Prahlada, by the strength of that vow, you came to render devotional service unto Me.
In her next life, the prostitute was born in heaven as an Apsara and after enjoying life there immensely, she entered My supreme abode. O Prahlada, you will also return to My abode and enter into intimate communion with Me.
In order to propagate devotional service to Me, you have incarnated without Me. After completing the task of preaching the glories of devotional service, you will again enter My eternal abode.
Those who observe this supreme vow in relation to Me will never return to this material existence, even in hundreds and millions of kalpas.
Because Lord Nrsirhhadeva incarnated on this auspicious Caturdasi, one should carefully observe fasting and then worship Lord Visnu in the evening. Because of My advent on the fourteenth day of the fortnight of the waxing moon in the month of Vaisakha, one should celebrate a festival on this sacred day, which destroys all sinful reactions.
This vow is generally observed on a Saturday, when the Svati-naksatra joins the auspicious day. Sometimes, by the arrangement of providence, there occurs a very auspicious moment known as siddhi-yoga. If all these factors combine then such a auspicious day can destroy even the sinful reactions for killing millions of living entities.
Even if the moment of siddhi-yoga does not occur, those who desire auspiciousness should still observe a festival on the fourteenth day because it was the day of My appearance. Still, Vaisnavas should never observe fasting if the Caturdasi is contaminated by the presence of Trayodasi.
If Caturdasi, which is very dear to the Supreme Lord, and which takes away all sinful reactions, falls on a Tuesday, it is to be observed. But even it is conjoined with Svati-naksatra if it is touched by the presence of Trayodasi, it should not be observed.
(The process for observing this vow) (It is also stated)
My dear Prahlada, on this auspicious day, one should rise early in the morning, brush his teeth, and then make a resolution to observe the vow strictly according to the prescribed rules and regulations, while constantly remembering Me.
(The mantra for making a resolution) O Lord Nrsimhadeva! O greatly powerful one, please display compassion toward me. To day, I will observe this vow of Yours and so kindly help me to complete it without any impediment.
Those who want to obtain the complete benefit of this vow should not indulge in talking with sinful people, nor should they engage in idle gossip.
On this sacred day, great souls who desire to observe this vow should give up conversing with women and playing chess [frivolous pursuits]. They should constantly remember My form throughout the day.
A learned person should then take his noon bath, either in a river or at home, or in a sacred kunda or a lake filled with lotus flowers, while reciting Vedic mantras.
While taking his bath, a devotee should rub his body either with cow dung, clay, amalaki fruit, or sesame seeds, which takes away all sins. Then, he should put on a fresh dhoti and chadara and carry on with his daily rituals.
After bathing and returning home while remembering Me with love and devotion, one should smear cow dung on the altar for worship and prepare a drawing of a lotus flower with eight petals on top of the altar. In the middle of the lotus one should install a copper pitcher decorated with jewels and place a container filled with rice paddy on top of it. On top of that container one should establish the Deities of Laksmi and Nrsimhadeva, made of gold.
One should give up all miserliness and use either one pala, one-half pala, or one-quarter pala of gold to make the Deities. The Deities should then be bathed with five substances before one begins to worship Them.
One should engage a qualified brahmana who is devoid of greed, well versed in sastra, peaceful, charitable, and self-controlled, as the priest. In accordance with the prescribed rules and regulations of the sastra, the priest will conduct the worship of the Deities. Intelligent devotees can also worship the Lord under the guidance of the priest.
On the sacred day of Nrsimha Caturdasi, which manifests in this world to destroy Prahlada’s distress, one should first offer worship to Prahlada before worshiping Lord Hari.
One should install My Deity and decorate Me with a flower outfit and then devotedly worship Me by offering Me seasonal flowers. One should offer sixteen ingredients for worship while chanting My holy names, as well as appropriate mantras. One should also recite mantras from the Puranas during the course of his worship.
O Supreme Personality of Godhead, Lord Nrsimhadeva, for your satisfaction, I now offer You divine, cool sandalwood paste mixed with camphor.
(The mantra for offering flowers) O Supreme Lord! O Nrsimhadeva, I worship You, along with Laksmidevi with offerings of flowers that bloom in spring, and tulasi leaves. My humble obeisances unto You.
(The mantra for offering incense) O Maha-Visnu, just to fulfill all of my desires, I will worship You with and offering of black aguru, which is rarely available, even for the demigods
O Nrsimhadeva, by offering this ghee lamp, which destroys the darkness of ignorance, as well as all sinful reactions, one obtains special power. Therefore, I offer this ghee lamp to You.
(The mantra for offering food) O beloved Lord of Laksmi, I place before You these offerings of food. May You be pleased with this offering and kindly remove all my sins.
O Lord Nrsimha! O Acyuta! O Lord of the demigods! O husband of Laksmi! O Lord of the universe, let all my desires be fulfilled by offering of this arghya to You.
(The mantra for offering worship) O Maha-Visnu, who wears yellow garments! O destroyer of fear for Prahlada, may You be satisfied with whatever little worship I offer You to the best of my ability, so that You bestow Your mercy upon me.
One should remain awake throughout the night while dancing, singing, playing musical instruments, reciting the Puranas, and listening to discussions of My transcendental glories.
Thereafter, one should bathe early in the morning and worship Me with love and devotion while following the above-mentioned procedure, without displaying any idleness.
O master of the demigods, please deliver all those who have taken birth in my family, as well as those who will take birth in the future, from the intolerable ocean of material existence.
O Lord who lies on the bed of Ananta Sesa! O Lord of the universe! O Nrsimha! O husband of Laksmi! O destroyer of the devotees’ fear, I am suffering from great distress due to drowning in the ocean of sinful activities and disease. So, kindly lend me Your fearless hand and give me protection.
O Janardana! O Lord who lies in the ocean of milk! O Supreme Lord, may You be pleased with my observance of this vow, and may You bestow upon me material enjoyment and liberation.
After worshiping the Lord and offering prayers to Him in this way, one should immerse the Deity into the water of the Ganges and then feed the acarya sumptuously.
Thereafter, one should bid farewell to the assembled brahmanas while giving them dakshine. Then, one should sit down and take prasada, along with his friends, while fixing his mind upon Me.
Link to original article: How Prahlada became a devotee of Nrsimhadeva