from Srimad-Bhagavatam 8.6

This is an abbreviated procedure for beginning devotees. It is actually much easier than it may appear on first reading. There is a video of Baba teaching this ceremony here. The complete Narayana-kavaca ceremony is given here [300 KB PDF].

Take full bath and then sit down facing north, wearing beads and gamsha. Offer obeisances to your spiritual master and to the Lord.

Chanting oṁ astrāya phaṭ, (“This is a spiritual weapon”) sprinkle water on the ācamana-pātra.

Now perform short ācamana as follows:

Sit gravely and silently, facing north. Chant your japa-mantra or Gayatri.

While chanting the 8-syllable mantra [oṁ namo nārāyaṇāya], beginning with the praṇava, (om), touch your hands to eight parts of your body: the feet, the knees, thighs, abdomen, heart, chest, mouth and head. Then chant the mantra in reverse while touching the parts of your body in the reverse order. Your intention is to put on the mantra like a suit of armor.
Then chant the dvādasakṣara-mantra [oṁ namo bhagavate vāsudevāya]. Preceding each syllable by the oàkära, place the syllables of the mantra on the tips of your fingers, beginning with the index finger of the right hand and concluding with the index finger of the left. Place the four remaining syllables on the joints of the thumbs.
Then chant the six-syllable mantra [oṁ viṣṇave namaḥ]. Place the syllable “oṁ” on his heart, the syllable “vi” on the top of your head, the syllable “ṣa” between your eyebrows, the syllable “ṇa” on the tuft of hair [śikhā], and the syllable “ve” between your eyes. Then place the syllable “na” on all the joints of your body. Meditate on the syllable “ma” as a weapon. Thus become the perfect personification of the mantra. Thereafter, adding visarga to the final syllable “ma,” he should chant the mantra “maḥ astrāya phaṭ” (“I am a weapon”) in all ten directions, beginning from the east. In this way, all directions will be bound by the protective armor of the mantra.

After finishing this chanting, one should think himself qualitatively one with the Supreme Personality of Godhead, who is full in six opulences and is worthy to be meditated upon. Perform bhūta-śuddhi by reciting the following prayer, meditating on its meaning:

“I am by nature the eternal servant of Kṛṣṇa, but by misfortune, due to being inimical toward Him from time immemorial, I have been identifying myself with the body and been continuously wandering in the cycle of birth and death in the material world, suffering the burning threefold miseries. Now, as a result of some unimaginable good fortune, by the mercy of my spiritual master I know that I am the eternal servant of Kṛṣṇa, that I am an infinitesimal spiritual being, completely apart from the gross and subtle body. Now, by the order of my spiritual master, following in his footsteps, I have obtained the good fortune of being able to serve his lotus feet and the lotus feet of Lord Caitanya Mahäprabhu and Śrī-Śrī Rādhā-Śyāmasundara. śrī-viṣṇu śrī-viṣṇu śrī-viṣṇu.”

Then chant the prayers to Lord Nṛsiṁhadeva:

namas te narasiṁhāya prahlādāhlāda-dāyine
hiraṇyakaśipor vakṣaḥ-śilā-ṭaṅka-nakhālaye

I offer my obeisances to Lord Nṛsiṁhadeva who gives joy to Prahläda Mahäräja and whose nails are like chisels on the stonelike chest of the demon Hiraṇyakaśipu.

ito nṛsiṁhaḥ parato nṛsiṁho yato yato yāmi tato nṛsiṁhaḥ
bahir nṛsiṁho hṛdaye nṛsiṁho nṛsiṁham ādiṁ śaraṇaṁ prapadye

Lord Nṛsiṁha is here and also there. Wherever I go Lord Nṛsiṁha is there. He is in the heart and is outside as well. I surrender to Lord Nṛsiṁha, the origin of all things and the supreme refuge.

tava kara-kamala-vare nakham adbhuta-śṛṅgaṁ
dalita-hiraṇyakaśipu-tanu-bhṛṅgam
keśava dhṛta-narahari-rūpa jaya jagadīśa hare

O Keśava! O Lord of the universe! O Lord Hari, who have assumed the form of half-man, half-lion! All glories to You! Just as one can easily crush a wasp between one’s fingernails, so in the same way the body of the wasplike demon Hiraṇyakaśipu has been ripped apart by the wonderful pointed nails on Your beautiful lotus hands.

Complete the ceremony by offering respectful obeisances to your spiritual master and the Supreme Lord.