The Mantra Sadhana can be commenced on any auspicious Tithi or festival like Diwali, Navratri, Dussehra, Sankranti, Holi or any Eclipse. On this day the Sadhaka has to purchase a New Photograph of Vishnu Bhagwan and keep it in your Puja-Ghar and worship it by offering it Dhoop, Deep, fresh flowers and a Bhog of Kheer-Puri. Then he has to garland the Photo with a Pearl Mala [Moti ki Mala].
Then the Sadhaka should take a Sankalp [pledge] to perform the full Om Namo Narayanaya Mantra Sadhana, which has been described below dutifully every day for 46 days.
The Mantra Sadhana has to be performed by chanting the Mantra 11,000 times daily for the first 45 days of the Mantra Sadhana. A Rudraksha Japa Mala can be used to keep track of the prescribed number of Mantra Chants.
Then on the 46th and final day of the Mantra Sadhana, Dashanash Havan has to be performed by offering Pure Cow Ghee to a Fire Havan, chanting the Mantra each time the Ghee is offered to the Havan. Dashansh for this Mantra Sadhana means 1/10 and hence the offering has to be done 1100 times.
This concludes the Om Namo Narayanaya Namah Mantra Sadhana. After the conclusion of the Siddhi Sadhana, the Mantra should be faithfully chanted 108 times daily.
Ladies can perform this Mantra Sadhana, but should take a break during their Menstrual Cycle.
Another method to gain Siddhi over this great Mantra is published in this post- Here.