The primary eight Siddhi are of a much higher order than the rest, and require severe discipline to accomplish. It is very rare that one will achieve such a perfection. But for every Siddhi there is a reflection that is easily attained. The processes for attaining these minor Siddhi are usually outlined in the Tantra Shaastra. [Please refer to the course on Vedic literature to understand what is Tantra Shaastra.  The processes generally involve doing Upaasanaa to a particular deity, who when pleased reveals their form to the Saadhak. On the way many Siddhi naturally develop due to expansion of the consciousness through Mantra Upaasanaa and meditation. According to the category of Devtaa one worships the result will come either quickly or after a long time, and the result will either be temporary or permanent. If you aim at a low entity, for example a ghost, the result will be quick, but it will be of minimal value. Whereas if your Upaasanaa is to a higher divinity, the result will be much more permanent and significant, but will take much more time to accomplish. The aim of the Saadhak generally depends on his conditioning within the modes of nature. This is described by Krishn in the Bhagavad Geetaa:

yajante saattwik devan yaksh-rakshamsi rajasah
pretan bhoota-ganams canye yajante tamasa janah

"Men in the mode of goodness worship the demigods; those in the mode of passion worship the Yaksh and Raakshas; and those in the mode of ignorance worship ghosts and spirits."

As you progress in the modes, the worship becomes more and more purified, from ignorance to goodness. When you finally transcend the modes by worship of Krishn, the worship is completely transcendental beyond the influence of material nature. The long term results of various Upaasanaa are also described by Lord Krishn :

yanti dev-vrat devan pitrin yanti pitri-vratah
bhootaani yanti bhootejya yanti mad-yajino 'pi mam

"Those who worship the demigods will take birth among the demigods; those who worship the ancestors go to the ancestors (Pitri Lok); those who worship ghosts and spirits will take birth among such beings; and those who worship Me will live with Me."

Sri AC Bhaktivedant Swami Prabhupaad, our divine spiritual master, says in his commentary to this verse:
"Pishaach (ghost) worship is called 'black arts' or 'black magic'. There are many men who practice this black art, and they think that it is spiritualism, but such activities are completely materialistic."

It should be noted that in this verse from Bhagavad Geetaa the first three types of worship are described as a vow (Vrat). The worshipper is making a vow to the object of worship in exchange for some material gain. An agreement is being made between the two parties. But in regards to the worship of Krishn it is stated to be devotion (mad-yajinah). There is no expectation on the part of the devotees. The true Yogee, meditating on the Paramaatmaa within his heart, has no desire for mundane mystical perfections. Their worship is completely unalloyed, without a tinge of desire for material enjoyment.

Another category of Siddhi involves the control of the object of worship. You do not directly acquire a Siddhi, but you maintain control over an entity who by nature of their higher existence possess natural powers. This relationship is maintained through your worship to the entity, who receives nourishment from the worship.

According to the level of the deity the results are greater and more permanent. But the greater the results the more difficult the process is. As you move up from the lowest levels of worship of ghosts, to Yaksh and Yakshinee, to minor Devtaa, the process becomes harder and harder (ie there is more sacrifice and sincerity involved). The perfection one achieves by worshipping a ghost cannot be compared to that attained by worshipping someone like Ganesh, but the worship of Ganesh will require more on the part of the Saadhak. The results will not nearly be as temporary as that attained by worshipping a ghost or spirit. In the same way, when you go beyond the worship of the Devtaa and you take up the worship of Lord Krishn, the Supreme Personality of Godhead, the perfection you will achieve will be much greater than anything that is offered within this material world. And more than that, it will be the most permanent (eternal to be precise). But at the same time, to attain it will require the most sacrifice and sincerity. From the bottom of the grades of worship up to the top most level (worship of Krishn) there is a direct correspondence between the necessary endeavor for perfection and the permanence of the result.

The third chapter of the Yog Sootra describe the following minor Siddhi :

The ability to know all languages including those of animals.
Knowledge of one's past lives.
Knowledge of the nature of other peoples minds.
The ability to make one's body invisible.
The ability to make the sounds of one's body unhearable.
The ability to make everyone happy and joyful.
The ability to possess great strength.
The ability to locate hidden things.
The knowledge of the fourteen planetary systems.
The knowledge of the arrangements of stars.
The knowledge of the movement of stars.
The knowledge of bodily anatomy.
The ability to remain motionless.
The ability to perceive the celestial beings known as Siddh.
The understanding of consciousness.
The knowledge of the soul.
The ability to walk on water, thorns and similar objects.
The ability to surround oneself with a blaze of light.
The ability to be omnipotent and omniscient.

How these powers are attained is summarized by Patanjali as follows:

janmaushadhi mantra tapah samaadhijah siddhayah

"The mystical perfections may be obtained either by birth, by elixir, by the chanting of mantras, by austerities, of by attainment of Samaadhi."

The Vedic texts describe 400,000 different species of humans existing throughout the universe (such as Yaksh, Raakshas, Vaanar, etc.) In many of the species, they are born automatically with various powers. This is the Siddhi attained by birth. By reciting certain Mantra and performing austerities one's consciousness is expanded and one develops supernatural abilities. Finally, by attaining the state of complete absorption in meditation, Samaadhi, one attains powers depending on the object of one's meditation. For example, one who meditates on the Sun gains complete knowledge of the planetary systems; one who meditates on the relationship between the ear and the ether attains the ability to hear anything.

After listing all of these apparently wonderful powers, Patanjali provides a warning:
tad-dvairamyadipi doshabijajakshaye kaivalyam
"By giving up even these powers the seed of evil is destroyed and liberation follows." This is the last test of the Yogee.
Patanjali mentions one final method for attaining these perfections: pratibhadwa sarvam - means "All these powers will come to one whose mind is spontaneously enlightened through purity."

Even without following a mechanical process of meditation, if one's mind is naturally purified by spiritual advancement and unalloyed devotion to God, one will automatically attain these various supernatural abilities. Lord Krishn confirms this in the Bhagavad Geetaa when He says:

The topmost Yogee is not someone who has artificially restricted his senses through mechanical processes (like Aasan, Praanaayaam, Pratyaahaar, etc), but one who has naturally engaged all of his senses in the devotional service of the Lord. The devotee, having experienced the spiritual bliss of Bhakti Yog, has no interest in mundane perfections and remains fixed on his ultimate aim to attain the lotus feet of Krishn. That is the ultimate perfection, sam-siddhi:

Questions and Answers:

The Saankhya system of philosophy explains how each element telescopes out from the previous element. The first physical or gross element is the Ether. Within the Ether all the other four gross elements are present (Earth, Water, Fire and Air) in a subtle form. The subtle quality of Ether is sound, and from the sound expands the Air. The subtle quality of Air is touch (movement), and from the touch expands the Fire. The subtle quality of Fire is sight, and from the sight expands the Water. The subtle quality of the Water is taste, and from the taste expands the Earth. And finally, the subtle quality of Earth, the last of the physical elements, is smell.

Thus you have a telescoping effect of the elements and their subtle qualities beginning from ether down to earth. Since each element is manifesting from the previous, each new element contains all of the qualities of the previous elements. The Ether element, being the first, only possesses the quality of sound. One cannot touch, see, taste or smell ether. The Air element, having expanded from the Ether, possesses both the qualities of sound and touch. One can feel and hear the movements of air, but one cannot see, taste or smell the Air. The Fire element's added quality is sight. Thus one can see, touch and hear the fire, but one cannot taste or smell Fire. The water, having expanded from the Fire, can be heard, felt, seen and tasted, but not smelt. And Earth, being the final element, contains all the five qualities of sound, touch, sight, taste and smell.

This sequence is the natural sequence of manifestation of gross matter. The Praapti Siddhi enables one, through Mantra, to reverse this process and transform gross physical objects into ethereal objects by merging the respective elements into their source element. Thus one takes an object located in a particular location, and by utilizing Mantra Siddhi, one merges the earth element into the water element, the water element into the fire element, the fire element into the air element, and finally the air element into the ether element. As you merge each element into the previous, the corresponding quality disappears (being merged back into the source element). In this way, a physical object is transformed into an ethereal object with no quality other than sound. This ethereal object can be transferred over space by mind, and then the entire process is reversed to re-manifest the original object in one's hand. What appears as a mystical feat to most is actually manipulation of the subtle laws that govern physical nature. There are eight major Siddhi and ten secondary Siddhi, all of which are based on manipulation of the subtle laws of nature.

To revive this latent Siddhi you will need to take up Mantra Upaasanaa very seriously. At present we are shifting our library from one location to another, so all of our manuscripts are in trunks. After one or two weeks, when the shifting is over, I will be able to provide you with the necessary Mantra and Yantra for Praapti Siddhi.

You must rekindle your spiritual pursuits. In your previous life you had undertaken much Saadhanaa. Now continue from where you left off and perfect your life. These Siddhi are not important at all. They will arise in anyone who takes seriously to spiritual practices. For the weak minded they are an obstacle on the path of self-realization. Their use is only to reaffirm one's faith that one is progressing on the path.

This verse is in reference to the present Brahmaa of this particular universe, who is a pure devotee of the Lord. It does not mean that every Brahmaa in every universe will automatically attain liberation. Neither will every inhabitant of Brahm Lok automatically go to the spiritual world. Brahmaa is no different than any other living entity. If he engages in pure devotional service he goes to the spiritual world. The same is the case for any of us. If he does not execute unalloyed devotional service, he will take his birth according to his desires.

Very rarely there is a Bahir-mukh Brahmaa, a Brahmaa who is averse to the Lord's devotional service. Generally all Brahmaa are favorable to Bhakti, but there are exceptions. Sri Viraraghava Achaarya has stated, based on revelation, that two Braahm Kalp (lives of Brahmaa) ago within this universe there was a Bahir-mukh Brahmaa. Otherwise we generally do not get information of what has occurred in other universes, or even within the same universe in prior creations.