The Narayana Kavacham occurs in chapter eight of the sixth skanda of Bhagavada Purana. It is an Armour to protect ourselves from our enemies seen and unseen.
Raja Parikshith, the son of Abhimanyu, asks his teacher, Sage Shuka for a means to protect himself from his enemies. Sage Shuka then teaches him the Narayana Kavacham, which was originally taught by Sage Viswaroopa, the son of Thwashtra, to Indra.
It is believed that the soul of the man who reads this Kavacha becomes extremely holy and that any one who reads this would be protected by Lord Vishnu. Sloka 4-11 of this great Stotra give us in detail about the preparations that we should take before reciting this kavacham. Kavacham's should be learned by a Guru who is well versed in these procedures and chanted under his guidance. If that is not possible, than the kavacham can be recited without performing the Kara and Anga nyasa or simply recite the last four lines. Before commencing the recital, chant ’Om Namo Naraayanaya’ or ‘Om Namo Bhagvadey Vaasudevaaya’ or your guru Mantra 12 times.
Visualize yourself and/or your loved ones, totally protected. You could ‘see’ them dressed in an armor. You could see them immersed in white or violet light. You could see them in a tube of Light. You could see an impenetrable ring of white Light around them.