Most siddhis is believed to be gained if you chant the mantra for 125.000 times at prescribed hours, correct pronounciations, and other requirements. That means hundreds if not thousands of hours sitting in your asana daily mentally chanting it until you find enlightment/siddhi (IMG:
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There's a good book by Chawdri that describes and details various siddhi attainment in his books Practicals of Mantras and Tantras, also his other books on mantra. You'll have to import the book from India.