The Yoga Sutras of Patanjali is a foundational text of Yoga. It forms part of the corpus of Sutra literature dating to India's Mauryan period (321-185 BCE). In Indian philosophy, Yoga (also Raja Yoga to distinguish it from later schools) is the name of one of the six orthodox philosophical schools.   Though brief, the Yoga Sutras are an enormously influential work on yoga philosophy and practice, held by principal proponents of yoga such as Iyengar (1993: p.xiii) as being of principal importance:
Pata?jali fills each sutra with his experiential intelligence, stretching it like a thread (stra), and weaving it into a garland of pearls of wisdom to flavour and savour by those who love and live in yoga.... 
The process westerners call "meditation" is called Samyama (synthesis of control. Here is what wapedia says about it:
Samyama (from Sanskrit sa-yama?holding together, tying up, binding  ). Combined simultaneous practice of Dhra (concentration, intent), Dhyna (contemplation) & Samdhi (unity). A tool to receive deeper knowledge of qualities of the object. It is a catchall term summarizing the process of psychological absorption in the object of meditation. 
Samyama, as Patanjali's Yoga Sutras states, engenders prajna. Adi Yoga or Mahasandhi discusses the 'mla praj?' of "listening/studying, investigation/contemplation, realization/meditation" which are a transposition of the triune of Samyama. These are activated subconsciously in non-structured form (thus producing fragmented spontaneous Samyama-like effects) by any thinking activity (particularly the Catuskoti and Koan  ) and deep levels of trance. Any kind of intuitive thinking at its various stages of expression is strongly related to Samyama-like phenomena as well. 
Practiced consistently by Yogis of certain schools (Raja Yoga, Adi Yoga e.g.).  Described in Yoga Sutras of Patanjali, it comprises the three upper limbs of Raja Yoga. Following Patanali's Yoga Sutras, a yogin who is victorious in samyama vanquishes all 'cognitive obscurations' (Sanskrit: klesha). The Sutras describe various 'powers' or 'perfections' (Sanskrit: siddhi) a yogin may attain through the conduit of Samyama. 
Samyama constitutes the total activity of meditation according to Raja yoga. It begins with deliberate concentration (dharana), evolves to effortless immersion of consciousness into being (dhyana) and culminates in the experience of ecstatic unity (samadhi) with the essence of existence underlying any object of meditation, abstract or objective. In the west meditation usually refers to the first stage of samyama, that of deliberate focus.
I suppose the other two stages aren't included because they are supposed to grow out of the practice of the first stage, and are not learned through instruction. There is no way to supply directions on how to perform dhyana (contemplative immersion) outside a loose description of the process, and certainly samadhi is completely beyond words.
The following material from wapedia may, however, give the seeker some idea of what these three dynamics are about:
Dhra (from Sanskrit dhra) is translated as 'collection or concentration of the mind (joined with the retention of breath)', or 'the act of holding, bearing, wearing, supporting, maintaining, retaining, keeping back (also in remembrance), a good memory', or 'firmness, steadfastness, ... , certainty'  . This term is related to the verbal root dhri to hold, carry, maintain, resolve.
Dhra is the sixth stage, step or limb of eight elucidated by Patanjali's Ashtanga Yoga or Raja Yoga. For a detailed account of the Eight Limbs, refer to the Yoga Sutras of Patanjali.
Dhra may be translated as "holding", "holding steady", "concentration" or "single focus". The prior limb Pratyahara invoves withdrawing the senses from external phenomena. Dhra builds further upon this by refining it further to ekagrata or ekagra chitta, that is single-pointed concentration and focus, which is in this context cognate with shamata. Maehle (2006: p.234) defines Dharana as: "The mind thinks about one object and avoids other thoughts; awareness of the object is still interrupted."
Dhra is the initial step of deep concentrative meditation, where the object being focused upon is held in the mind without consciousness wavering from it. The difference between Dhra, Dhyna, and Samdhi (the three together constituting Samyama) is that in the former, the object of meditation, the meditator, and the act of meditation itself remain separate. That is, the meditator or the meditator's meta-awareness is conscious of meditating (that is, is conscious of the act of meditation) on an object, and of his or her own self, which is concentrating on the object. In the subsequent stage of Dhra, as the meditator becomes more advanced, consciousness of the act of meditation disappears, and only the consciousness of being/existing and the object of concentration exist (in the mind).
Dharana is the easiest stage of meditation to understand. Even so, it constitutes the sixth stage of an even longer suggested preparation to attain ultimate unity with the essence of consciousness. The key to practicing concentration is in the interpretation of the sankrit term dharana. It refers to a gentle yet steady holding of attention. There is no stress or strain here as might be implied with the idea of "one-pointed" focus. Focus here is held like a feather in the palm of the hand. Very lightly and steadily.
Westerners usually begin practice directly with concentration, whereas in the east preparation for this is advised. These preparatory stages actually qualify dharana and samyama in general differently than what is common esoteric knowledge in the west. Preparation begins with control, a word synonymous with "death" as in death of ego habits. The first stage of yoga refers to proper actions, and is a positive focus of behaviour. The second stage refers to improper actions and represents behaviours to avoid because they promote ego habits and a conditioned mindset.
One may argue against this, however, as Tantriks often do because behavioral control, even self-imposed, may just be another form of conditioning.
Be that as it may, the idea is to be aware of habits that represent programming. The whole idea of yoga is to be liberated from such programming so that our consciousness can discover its true nature. So the first step is to deal with our mental programming, those actions that are compulsive and do not really represent free will, although that is how it may seem on the surface. This may include dietary cravings, knee jerk reactions to what people say and do, a whole number of ideological assumptions, and what Christians refer to as the seven deadly sins.
The latter from a yoga perspective are considered to be the result from identifying with our assumptions. We get angry, or prideful or lazy or lustful because of what psychologists call Pavlovian conditioning and identification with persona programs. Looking deeper into this, anger, self-esteem, passion and even relaxing from work are not bad in themselves when natural responses of our being. Most of the time, however, these responses are based on embedded assumptions in our minds, defensive attitudes those assumptions generate and neurotic insecurities festering from deep psychological imprints.
The first step is, therefore, to watch what we do and how we respond and be very very honest with ourselves without condemning or justifying. We identify the neurotic pattern or assumption and release it, and do this constantly and consistently. However, this is not enough. Programming can also be locked in our tissues.
Thus the third stage, called asana or posture, actually refers to what western therapists call "body-work". There are methods that interpret body posture in terms of psychological imprinting. The body holds neurosis in the tissues through muscular armouring reflected in chronic tension and lack of balance in posture. Yoga solves this through applying stretches to relieve this tension, but massage and specific exercise targeted toward relieving armouring also works. Ideally all three approaches (stretching, massage and exercise) are most effective.
Once the body musculature and connective tissues are addressed in this manner, the fourth stage of yoga can be brought into play. This is called pranayama or life-force control, done through engaging in specific breathing patterns. The word prana is equivalent to the Oriental word Qi or Ki signifying the sensations that can be called "energy" generated through patterns of breathing and conscious attention.
When we combine breathing with conscious attention we notice corresponding patterns of sensation in our bodies. These patterns of sensations are life flows of energy, and this is how we usually refer to them. Once we are behaviourally deconditioned toward self-honesty and conscious instead of knee-jerk responsiveness, and once our major psychosomatic imprints are addressed and well on the way to being released, we are in a position to cultivate the flows of sensation that prepare our body/minds for the meditative experience.
Once we feel what flows within, we move to the next stage pratyahara, which is detatching from the five senses to amplify internal experience. In the west this stage is usually bypassed, and concentration via the five senses is engaged. In Raja yoga, however, the idea is to be tuned into our sensate responses experienced as energies flowing with us rather than the responses to outer sensory stimuli. In doing this we are on the path to Self-realization, as opposed to being locked onto sensory appearances.
Then we move toward active concentration or dharana. The preparation allows us to be objective in our subjective experience, and to sense every response of consciousness our concentration engenders. Without preparation concentration is full of habitual distractions from body and mind, and our response habits will very likely mask the necessary sensitivity to flows of sensation that conentration stimulates. This can make concentration a much more difficult experience than it otherwise would be.
Dharana, as I said, evolves toward dhyana or contempation proper: In Hinduism, dhyana is considered to be an instrument to gain self knowledge, separating appearance from reality to help attain the ultimate goal of liberation from the shackles of perceptual conformity. Because our attention is internalized, and our awareness is free of conditioned impulses, our attentive awareness can relax and stabilize into an effortless immersion that goes beyond any particular object of focus and simply experiences the nature of consciousness.
This is like watching the waves of the ocean to eventually get a sense of the ocean itself independent from all the waves it makes. At first, because we are conditioned to respond to the waves, we don't even recognize the ocean or only peripherally so. We think the waves are independent entities clashing with each other. But as we relax more, we can sense deeply the watery nature they have in common and observe that their movements are due to the whole of the medium that gives them existence.
The same is true for consciousness. With contemlative immersion we reach a state where we experience consciousness not as an isolated individuality responding to independent forms around us, but as a continuum that permeates and gives existence to both us and the world around us. Although in itself this is a profound revelation, it is not liberation per se, because we are still standing back from the experience, still separate from it.
Although we are now in a state that sustains itself because it has become natural for us, we are still observers and experiencers of it. Samadhi takes our immersion to the next level, and evolves naturally from it. This is where we notice the ocean of being around us as being one with the ocean of being not only within us, but US. This take immersion to the level of complete identification with the medium of experience underlying phenomena.
This is a tremedously pleasurable state and results in being able to interact with reality in a magical way because our relationship with phenomena is based on the reality underlying appearance, rather than taking appearance at face value. We are now truly deprogrammed, as if our life were some cult or oppressive dogma from which we have managed to escape and come home.
For Raja Yoga this is the goal. However, one may realize at this stage that this is only the beginning.