How is mantra siddhi done

How can one achieve "Siddhi" in Hinduism?

There is actually more than one question in your question namely:

1. How many types of siddhis are there?

2) How do you get there?

3) What is Mantra Siddhi and how can it be achieved?

How many types of siddhis are there?

The Linga Purana describes the primary siddhis as follows

A successful yoga practitioner is with 'Dasha Siddhis'viz equipped.Pratibha, Shravana, Vaarta, Darshana, Aaswaada and Vedana. Jnaana Pratibha is the siddhi of knowing about a vastu or vyavahar or about something of the past, present or future; This siddhi would influence or impress others. Shravana Siddhi enables a yogi to hear or understand any type of sound or conversation or event, regardless of distance or time. Vaartaa Siddhi facilitates the Pancha Tanmatraas experience with Sabda-Sparsha-Rupa-Rasa and Gandhas. Darshana Siddhi is the ability to perceive or see things regardless of time and distance restrictions. Aaswaada Siddhi enables the tasting of any substance and Vedana Siddhi is the power of sparsha to experience the form, shape or characteristic of a person or thing.

Other types of siddhis are described on the same page as follows:

Indeed, a Maha Yogi is supposed to be possess sixty-four kinds of Paisachika, Parthiva, Raakshasa, Yaaksha, Gaandharva, Aindra, Vyomatmika, Praajaapatya and Brahmaadi Siddhis, but he should abandon all these powers in search of Shivatwa. Such siddhis range from adopting any kind of swarupa with obesity, slimness, childhood, youth, old age, male, female, any kind of bird-animal-reptile, mountain, body of water, and so on; Ability to lift mountains, drink an ocean, fly in the sky, walk through the eye of a needle, and endless such wonders. [ The normal siddhis are anima (miniaturization), mahima (maximization), prakamya (seeing and hearing), isatwa (dominion), vasitwa (self-control), kama vasitwa (ability to grant wishes), doora shravana and doora darshana or distant vision and hearing, Parakaya Pravesha, or the ability to enter the bodies of others;'Devaanaam Saha kreedanam' or playing with devas, Yatha Sankalpa Samsiddhi or instant fulfillment of wishes, Triloka Jnaana or knowledge of the events of Three Lokas;Control of heat and cold, Paraajaya or invincibility and so on. ]]

How do you attain siddhis?

The way to get Siddhis is to practice "Ashtanga Yoga". The description of Ashtanga Yoga is on the same page where I quote:

Controlling the activities or processes of the mind is yoga.Ashtanga or eight limbs yoga consists of Yama-Niyama-Aasana-Pranaayama-Pratyahara-Dharana-Dhyana-Samadhi . Yama is characterized by good behavior that includes ahimsa (non-violence), satya (truth), asteya (theft), brahmacharya (avoidance of sex), and aparigraha or excessive acquisition. Basic Niyamas is in fact an extension of Yama: it is ahimsa not to injure or torture others, either physically or mentally. conveying what is seen, heard and believed is satya unless it does not hurt any body is known as satya; Refraining from stealing the property of others by deed, thought, or otherwise is called asteya; to negate from relationship with another woman or in relation to a woman with another man, other than the married woman or man, and this should be observed by seeing, thinking, or acting is called brahmacharya or celibacy; and to refrain from short-term procurement of materials that are far beyond one's own needs is known as aparigraha. In addition to the above general principles, Niyamas also includes the specific acts, viz. Shoucha or physical cleanliness, yagna tapa or meditation, daana or charity, swadhyaya or memorizing or reciting the scriptures, indriya nigraha or control of the limbs, vratas such as chandrayana as well or to perform formal worship directed to specific devatas and purposes ,,

What is mantra siddhi and how can it be achieved?

Mantra Siddhi is a devotee's full control over the healing power of a mantra and is obtained when a particular mantra is recited for a set number of times with certain other restrictions being imposed. First of all, you should get your guru to receive a mantra. Or else you can pray to your Ista Devta and begin. The rest is quoted from this site. Mantra Siddhi is also called Mantra Purasarchana.

METHOD OF MANTRA PURASCHARANA

The repetition of a mantra with rigid spiritual observations a set number of times in order to achieve rapid spiritual progress is known as the mantra purascharana. It can also be done for material advancement. The practitioner should follow certain rules and undergo strict dietary discipline to ensure a quick mantra siddhi.

During the Purascharana, only take fresh vegetables, fruits, milk, roots, barley and Havis-Anna (rice cooked with ghee, sugar, milk). A sadhaka can also live on pure bhiksha (alms). If you can live on milk alone during the time of Purascharana, it is very commendable. You can have mantra siddhi by repeating the mantra a few times.

Choose a sacred pilgrimage site on the banks of St. Ganga, a confluence of rivers, mountain valleys with enchanting landscapes, temples and Tulasi gardens under Asvattha trees, or turn part of your home into a temple by burning the image of the Lord with incense etc. and through appropriate decorations. Purascharana performed in holy places is a hundred times more useful than what is done in one's own home.

You can choose any mantra for Purascharana. Your Guru Mantra or Ishta Mantra is the best. Sandhya time, sunrise, sunset, and noon are recommended for Japa. Repeat the mantra as many times as there are letters in the mantra. You can do half that number. In no case, the number should be less than a lakh ........

After completing the Purascharana, perform homa with the number of japa, tarpana (water libation) with the number of homa, marjana (spraying) with the number of tarpana, and feeding brahmins with the number of tarpana. You can do feeding and charity activities according to your capacity if you cannot strictly adhere to the above regulations.

It is recommended that you read the entire article and not just the parts I cited from it.