According to the Tantric texts, to attain Mantra Siddhi you have to
1. Repeat the Mantra a specified number of times. Normally the number of letters in the Mantra multiplied by lakhs. For example for Navakshari (Navarna) Mantra it is 9,00,000 times.
2. Perform a Purascharana homa. The number of Ahudhis should be 1/10th of the repetitions. For Navakshari it is 90,000.
3. Tharpana of 1/10th of Ahudhis. Thus for Navakshari it is 9,000.
4. Feed number of Brahmins 1/10th of Tharpana. For Navakshari it is 900.
If you have done all this you will get the Mantra Siddhi.
Mantra Siddhi is important in the case of Krama Deekshas where you have to get the Mantra Siddhi of one Mantra before proceeding to the higher one.
But then let me raise a question which may seem basic, but which is not answered.
There is no book which defines Mantra Siddhi.
1. What do you think is Mantra Siddhi?
2. What do expect when you get Mantra Siddhi?
Fly through Air? Attain Ashtama Siddhi? Have Dharshan of the Mantra Devata? Some might talk about Kundalini reaching the Sahasrara or the Crown Chakra. Then what happens?
Are we aiming for something about which we know nothing?
Is Mantra Siddhi necessary for spiritual advancement?
Jai MAA KAALI !!!