Goswami Tulsidasa writes in a certain legend in Ramayana, an ascetic tells King Pratapbhanu upon the latter being amazed to see the occult powers of the adept:

Jani ācaraju karu mana māhi, suta tapa tē duralabha kachu nāhi.
Tapabala tē jaga srjayi bidhātā, tapabala biṣṇu bhayē paritrātā. 
Tapabala sambhu karahi sanhārā, tapa tē agama nahi kachu sansārā. (Tulsi Ramayana, 1.162.1-2)
Do not be surprised dear. Nothing is impossible with sadhana. It is with tapas alone that Brahma creates, Vishnu protects, and Shiva destroys. With penance, nothing in the three worlds is unattainable.
There are no quick fixes. Success in sadhana, practice, of self-transformation, is proportionate to the amount, quality, and consistency of efforts put in. Great persistence is rewarded by extraordinary rewards.

In aphorisms of Patanjali, he states that yoga is the practice of erasing your psychic imprints in Yōgaścittavrttinirōdhaḥ (Patanjali Yoga Sutras, 1.2) and he starts by saying atha yōgānuśāsanam (ibid, 1.1), “Now begins the discipline of yoga”. So, sādhāna is about adhering to the specified discipline; in a strict and careful manner till you attain your goal.

The yoga of self-transformation helps the practitioner to turn inward completely. Besides discipline, the success in the actual sadhana depends on four key aspects: sadhaka (aspirant), siddha (master), sadhya (goal) and sadhan (resources). The absence of any one of the four puts the whole practice on a shaky ground. The palace of stones turns into a sand castle — fragile and temporary. It is worthwhile to understand the fundamentals of sadhana before delving into the actual practice. As follows:

1. Sadhaka — The Seeker

The seeker is the sadhaka (साधक), practitioner, who is committed to the path. He has realized the futility of material pursuits and naturally builds upon on the flourishing indifference (vairāgya) towards material attainments. He has understood that irrespective of the degree of success in worldly pursuits, they continue to be temporary. Further that, they, ineluctably, bring greater misery and fear. In the material world, the first fear is of failure when you are working towards creating something. If you succeed, the second fear is of losing your acquisition.

Protecting and holding on to the achievement keeps one on one’s toes. A seeker is the one who has decided to go beyond his fears and cross the ocean of his tendencies of the mind to reach the peaceful shore of inner peace and his true nature. There is no practice without the practitioner. The seeker forms the main pillar of the actual practice.

The seeker must be committed to two things above everything else: practice and dispassion.

Krishna says to Arjuna:
 Asanśayaṁ mahābāhō manō durnigrahaṁ calaṁ
 Abhyāsēna tu kauntēya vairāgyēṇa ca gr̥hītā.(Bhagavad Gita, 6.35)
 O Warrior! Doubtlessly, it is an extraordinary feat to conquer one’s mind. But, Arjuna! With practice and dispassion it is possible.

If the aspirant continues to find joy in materialistic or sensory pursuits of the external world, his journey to discover the inner one will be a drag. His efforts are going to be diluted. Dispassion comes as a result of reflecting on the temporary, illusory, and selfish nature of the material world as well as with the conscious effort of letting go (tyāga) all that binds you. A genuine seeker has both.

If you can enjoy with no attachments, you have learned the art of living. It is not the joy itself but the attachment to such joy that binds one. Detachment comes from staying even under all circumstances. And evenness comes from an ever-present mind that helps you remember to stay course. An alert mind, in turn, is a natural outcome of good meditation.

2. Siddha—The Guru

The guru is the siddha (सिद्ध), adept, who has already been there and done that. His role is to guide the seeker and help such practitioner get across the hurdles. Since eons, all systems of various yoga and the practices thereof have been passed on from guru to the disciple. To the highly committed seeker, finding and meeting his guru becomes an automatic act — almost like a preordained rendezvous. There is no need to go out in search of a guru.

Once you start the actual practice and stay on course firmly, Nature will arrange for your guru to come into your life. And till such time even if you do meet the right guru, you will stay oblivious and unaware. Do not be hasty in selecting a guru. Guru is not a formality or a transaction of convenience. He is not an item you can just tick off.

The right guru can dispel your doubts, help you cross the hurdles, and assist you in spiritual attainment faster and better than you on your own.

abhavētsaṅgayuktānāṁ tathā viśvāsnāmapi,
gurupūjāvihinānāṁ tathā ca bahusaṅginām.
Mithyāvādaratānāṁ tathā ca niṣṭhurabhāṣiṇām,
gurusantōṣahinānāṁ na sid'dhiḥ syātkadācana. (Shiva Samhita, 3.17-18)
The one who is attached to the material world, lacks faith, has no devotion towards his guru, rejoices in socializing, lies, speaks harshly, disregards guru's needs is unlikely to attain his spiritual goal.

A guru is not indispensable. I am deviating from the word of the scriptures with the previous statement; I am doing so based on my personal experience and observations. However, the right guru can certainly accelerate your journey. If you lack the burning desire, even God cannot help you attain your goal, let alone any guru in the human body. Just like a competent spiritual master can bring out the best in his disciple, a devoted disciple can also get the best from his guru.

Devotion, faith, honesty, service are the ingredients of the right recipe. If your guru wants material things from you or if his giving time to you is dependent on how much you have to offer, forget that guru like a bad dream. How can the one who has not risen from desires himself instruct you on dispassion?

Guru banāyō jāni kē, pāni pīyō chāni kē, Drink water after filtering and make guru after examining, a famous saying in the Nath sect. It has traveled verbally so I am unsure of its origin.

3. Sadhya — The Goal

The third important pillar is the goal, sadhya (साध्य). It is pivotal to know your target. The yoga of self-transformation has many milestones that are aimed at guiding the seeker. If you do not know your goal, attainment of anything will mean everything, or achieving everything may mean nothing. The course of action can only be charted if you know the destination.

Some may want occult powers, others, Samadhi, and many others may just want a better way of living. Anything is possible with the right practice. For a yogi, the goal may be a yogic trance. For the mantrin, the goal may be to invoke the latent power of the mantra. For the worldly, it may just be acquisition of material wealth. For the ill, it may be to regain his physical health. Whatever your goal may be, it is important to have one and carry out the practices accordingly.

I would like to quote a verse from a vaishanava tantric text:

Āradhāitō yadi haristapasā tataḥ kim 
 Nārādhitō yadi haristapasā tataḥ kim.
 Antarvahiryadi haristapasā tataḥ kim
 Nāntarvahiryadi haristapasā tataḥ kim.(Nārada Pancrātra, 1.2.6)

If your worship leads to Śrī Hari, what good is any penance; of what use is any austerity if you are not lead to Śrī Hari. If you worship him within and without, tapas is no longer important; if you do not worship him within and without, how can tapas be of any importance.

Read the above statement multiple times to let its profundity sink and settle. Although uttered with the devotional sentiment, the aforesaid statement highlights the essence of all sadhana. That is: one must never lose sight of one’s goal on the path of sadhana. If you are treading any path that does not lead you to your goal, why waste your time on such a path? The rules, discipline, framework, and philosophy are only means to an end. Your sadhana is to attain your sadhya. Once done, you are ready to define your own rules because when you are delivered, you are free. Free of everything.

It is important to follow the rules initially. Just like if you are driving on the highway, for your own safety and that of others, you follow the traffic rules. But once home, you no longer walk in lanes or indicate before turning. Very much like the lane markings are there to guide you, any discipline on the spiritual path is to assist you. If you turn your discipline into a ritual losing sight of the ultimate, your path will become a conglomerate of dry practices devoid of bliss and joy.

4. Sadhan — The Resources

The last and certainly not the least aspect is sadhan (साधन), resources. It is not possible to do the practice without support. By resources, I do not mean that you require substantial financial capital. For the initial stages, you can start the practices sitting at your home. As you progress, your circumstances should allow you to do almost like a “meditation getaway” of a week, ten days, or month once or twice a year at the minimum. Your routine during that getaway will be intensive meditation.

Further, with continued advancement, a “quieter” place will be required for intensive practices of silence and solitude. You are not going to see serious results without an intense practice.

You need not worry about any of the above at this stage. To begin with, you require a burning desire, commitment to the practice, purity of moral, spiritual, and yogic discipline. For the genuine seeker, the providence arranges for everything as the need arises. I say this from my first-hand experience.

Enjoy the journey!