Śaṅkara generally describes upāsana as: Sitting close (upa) (āsana) to the meditation object (upāsya). What is close to the meditation object? The mind (manas). What is the object? All objects are īśvara, which is all power, all knowledge. Upāsana yoga therefore means understanding what īśvara is, the whole, and that the jīva, the individual living being, is a part of it. This practice, along with karma yoga, will prepare the mind to realise how you are equal to īśvara, as the common carrier of both: Consciousness, Brahman.

Whereas karma yoga is the activity (karma) of the whole being (sarvabhavena, the subtle and gross body), upāsana yoga is the activity (vyāpāra) of the mind (manas) or, more broadly, of the whole subtle body, the gross body being merely the activity of the instrumental brain serving the subtle body.

Upāsana yoga is therefore formally defined as saguṇa brahma viṣaya mānasa vyāpāraḥ, a mental (mānasa) activity (vyāpāraḥ) whose object (viṣaya) is saguṇabrahma (consciousness with qualities).

Both (karma and upāsana yoga) lead you through the sacred concept of īśvara, to knowledge of the infinite self. This is done by realizing that all apparent qualities of reality, caused by the mithyā force of māyā, belong to īśvara. What is the function of this?

These activities release loving devotion (bhakti). This attitude makes me calm and sharp. Then I see that body and mind are part of the living being (jīva), which is part of all minds and bodies (īśvara).

Because I have a very limited freedom of choice in life and world, and see that īśvara controls everything, I discover that I cannot be jīva. Then I am qualified. If knowledge is then imparted to me, I will recognize my full being.

A verse that encourages upāsana yoga and announces self-knowledge is Bhagavad Gītā 12.8:

mayyeva mana ādhatsva mayi buddhiṃ niveśaya |
nivasiṣyasi mayyeva ata ūrdhvaṃ na saṃśayaḥ ||

"Focus your mind completely on me, let your intellect rest in me. If you do that, you will definitely abide in Me, without any doubt."

Meditation is eminently also training focus, as one focuses the mind one-pointedly (ekāgratā) on a subject.

Upāsana and dhyāna are themselves synonyms in the sense of meditation. Although the term upāsana is used somewhat more specifically for Vedāntic meditation (which also involves, for example, employing pūjā or mantras without reciting aloud), and dhyāna is partly known as a term for all types of meditation.

Upāsanam reduces mala (impurity of mind) and vikṣepa (agitation, projection, restlessness and extroversion of mind).

Karma yoga (along with possibly practising aṣṭāṅga-yoga) purifies the mind. Combined with upāsana yoga, one-pointedness and expansion of the mind is achieved. When used as a preparatory tool for self-knowledge, upāsana is practised to develop sufficient focus and subtlety of mind (by reducing mala and vikṣepa) to hear and understand the teachings properly in śravanam. And to clearly reflect (manana) and contemplate (nididhyāsana) on the knowledge.

What is the knowledge?
Seeing everything as bhagavān, the lord is upāsana or dhyāna. Knowing that everything is bhagavān, equal to myself is jñānam, knowledge. Conclusion: There is only bhagavān, there is only self.

Upāsana (and karma) yoga are thus necessary to be fullness, freedom, singleness, infinity through knowledge.

To conclude. Two types of upāsana, meditations are technically described. The first is dual, and a preparation for self-knowledge, the second already goes a bit more towards nididhyāsana.

I. Sampat upāsana. This type of meditation has been described so far: Meditation (upāsana) on something great, on something abundant or beneficial (sampat). In this form of meditation, a particular object is regarded as more than it is.

The object can be a flower, a stone, a carved form (a murti), it can be (anything). Whatever the chosen object is, the mind regards it as īśvara. In this way, something greater is superimposed over an ordinary object. This way of looking, this contemplation, is the basis of much worship and prayer in India. In the river, in a tree, in a stone, anything can become an object of my devotion.

Actually, we cannot even say that a person is a piece of god. For God is one, non-dual. The manifestation of the divine is also one, namely one big system, to which everything belongs. If you take away one so-called particle, then everything is gone, because there are no parts. This is hard to imagine.

But seeing everything as consciousness or god is right. How then? The relationship of an object to consciousness is not one of time, space or dimension. In fact, consciousness is not time, space or dimension. The relationship between a partial object (vyaṣṭi) and the totality (samaṣṭi) is one of substance!

Every object turns out to be nothing else consciousness, and that has no dimensions, nor parts. That is the truth behind īśvara. We can peel off an object to consciousness, and that is well ]undivided (akhaṇḍa). We can see a wooden table as wood, as molecules, as atoms, as quantum particles, as information, as knowledge, as pure knowledge, as consciousness. There is only one consciousness, and it is not space or time or matter. So everything I see is consciousness in the same way. In manifestation, a partial object is a part of the cosmos, or a thought is a part of the universal mind (hiraṇyagarbha). But in reality, there is no division or subdivision.

What is the stunning conclusion here: the one who knows sees, in every object, the infinity of himself. This can be meditated on. And this makes meditation on an object, a burning candle or so, very exciting.

This seeing the infinity in everything is somewhat different from actually seeing sensory everything at once, of course. This occurs characteristically in mystical visions or in Arjuna's viśvarūpa vision in the Bhagavad Gītā. But such a (psychotic) vision is not at all necessary to know that everything is singularly divine.

In the story The Aleph by Jorge Luis Borges, Borges describes that the first-person 'knows' that there is a point in the basement of the house he visits in which everything is. I quote: 'At that one point I saw, without any distortion or overlap, the entire universe: the great and the small, the sky and the earth, all the stars, all civilisations, and also my own face, in that same instant in which I looked at the Aleph.'

Parallel to this is the viśvarūpa darśana yoga (The Yoga of Seeing the Universal Form) of Arjuna in Bhagavad Gītā Chapter 11. In this chapter, Śrī Kṛṣṇa reveals his Viśvarūpa (universal form) to Arjuna, in which Arjuna sees the entire universe, with all worlds, gods, time, death and infinite manifestations, simultaneously.

Verse 11.13 describes Arjuna's experience of the all-encompassing vision:

sa tatraikasthaṃ jagat kṛtsnaṃ pravibhaktam anekadhā |
apaśyad devadevasya śarīre pāṇḍavas tadā ||

There the son of Pāṇḍubin saw the body of the God of gods (Kṛṣṇa), the whole universe united in one place and divided into many forms.

As is well known, when he saw the beautiful, he was filled; when he saw the terrible, he was in horror and wanted to get rid of it.

II. Ahaṅgraha upāsana. Meditation (upāsana) in which the meditator shifts his identification from the individual self to the universal self (ātmābrahman). Literally "grasping (graha) of the (true) self (aham).

What you are practically doing is naming all aspects of the jīva as brahman. My body, my hand is seen as brahman, my body, my mind, everything I used to call 'me' is seen by me as brahman. My life, my environment, the others around me etc.

This is how you shift your identity from little me to 'just me'. Thus, ahaṅkāra and brahman are equated. Only this will not happen overnight.

Because every meditation is an activity, at this level you are still a doer, an ego, ahaṅkāra.
In this sādhana (exercise), ahaṅkāra, ego, is probably not yet properly ignored. So it is not a true nididhyāsana (contemplation).

In nididhyāsana, the opposition between meditating (doer) and what is being meditated upon (the done) is nullified. So knowing that I am brahman, the only thing, is different from seeing the person as brahman.