Mantra and its Vedic position.

Mantras for Japa should be Vedic origin. Mantra should be originated from a Yogi who attained Antakarana Suddhi. Mantra comes from his  intelligence when he unites with the universal conscience. In his real time he is selfless and no expectations born or come back in him.

Shiva is like that. Shiva eliminated his all passions with his third eye during his Nataraja Nrutta. Seekers who wish to learn something, students, Yogis are meditating on Shiva. Om Namashivaya, is also known as Taraka Mantra.

Due to delusions, many mantras are using nowadays for Sadhana. Sadhana kandam mentions the worshipped shield be passionless and unattached.

Example, Sri Vishnu, His Avataras like Rama and Krishna were meditated on Shiva. Shiva is Aadi Yogi. No one will be willing to give or share their Moha with others.
So Vedas gives methods of Sadhana in Sadhana Kanda.

Let us understand the meaning of Antakarana Sudhhi.

In Hindu philosophy, antahkarana (Sanskrit: "the inner cause") refers to the totality of two levels of mind, namely the budhi, the intellect or higher mind, and the man as, the middle levels of mind which (according to theosophy) exist as or include the mental body. Antahkarana has also been called the link between the middle and higher mind, the reincarnating part of the soul.

In Vedāntic literature, this antahkaraṇa (internal organ) is organised into four parts.

1. Ahamkara (ego) – identifies the Atman (self) with the body as 'I'
2. Budhi (intellect) – controls decision making
3. Manas (mind) – controls sankalpa (will or resolution)
4. Chitta (memory) – deals with remembering and forgetting

Another description says that "antahkarana" refers to the entire psychological process, including mind and emotions, are composing the mind levels, as described above, which are mentioned as a unit that functions with all parts working together as a whole. Furthermore, when considering that mind levels are bodies, they are: manomayakosha - related to manas - the part of mind related to five senses, and also craving for new and pleasant sensations and emotions, while buddhi (intellect, intelligence, capacity to reason), is related to vijnanamayakosha - the body of consciousness, knowledge, intuition and experience.

Pancha Mukha Rudra.

According to Śaiva Agama, Lord Shiva performs five actions - creation, preservation, dissolution, concealing grace, and revealing grace. Each of the five actions corresponds to a name and form of Shiva with varying attributes.
Names with Relation to Action Performed, Direction Faced, and Associated Element:

Sadyojāta - Creation. West. Earth. Pṛthvī.

Vāmadeva - Preservation. North. Water. Jala.

Aghora -Dissolution/Rejuvenation. South. Fire. Agni

Tatpuruṣa - Concealing Grace. East. Vāyu. Air.

Īśāna - Revealing Grace. North-east. Ether (Ākāśa).

Aspects l

Though bearing each a different name, form, and set of qualities, these are all aspects of a one Supreme Being - Śiva, and are not to be looked upon as different deities.

1. Sadyojāta: Represents Icchā Śaktī. This face of Śiva will give both happiness and sadness to all creatures. This face of Śiva can potentially evoke curse and anger from Śiva. Represents Jalandhara Pīṭha. One billion (100,00,000) mantras are trying to describe this face of Śiva. White in color. Ahaṃkāra element representing perfected ego. The fearsome aspect. This aspect is attained by solitude and practices that transcend conventional structures.

2. Vāmadeva: Represents Citta rūpa and Citta rūpiṇi of Śiva. This is Turīya, attained by getting acquainted with primordial energy of the sun. This face of Śiva has special powers to heal both mentally and physically of any creature. Represents Parāliṅga. Two billion (200,00,000) mantras are trying to describe this face of Śiva. Blood red in color it represents unmatched force that is capable of transforming all elements of the cosmos. Uplifts the element of Tejasa. Direction is North. Predominates the energy of vital life force. It represents indescribable amount of brightness of light. Only those established in yoga can contain it within their physical forms, otherwise the mortal frame sheds itself immediately resulting in union with Vamadeva. The adepts contain energy of creation of elements within themselves.

3. Aghora: Represents Jñāna Śaktī(Infinite Knowledge). It is function of Prakṛti (nature, consort of Shiva) and Parā Śaktī. This face of Śiva is Buddhi rūpa (Intellect). Represents Pūrṇagiri Pīṭha. Banaliṅgam. One billion (100,00,000) mantras are trying to describe this face of Śiva. Smoke(Dhumra varṇa) in color. It represents our balanced aspect of Ahaṃkāra Tattva (our ego nature). Mixture of Prāmaṇa and Prameya. It represents the forces of Rudra.

4. Tatpuruṣa : Represents Ānānda Śaktī. East direction. Kāmagiri Pīṭham. Manorūpa. It represents structure of soul. The individual merges with infinite. Two billion (200,00,000) mantras are trying to describe this face of Śiva. Yellow in color. Pṛthvī (Earth) Tattva. Svambhuva liṅga. If you have severe difficulty in focusing on any subject, you should meditate this face of Śiva. Śiva in this face meditates. Direction is East.

5. Īśāna: Represents the Citta Śaktī of Śiva. Sāmbā Pīṭham. Represents Space. Starts from your Mūlādhāra to Anahata to Ajna to Sahasrara leading to Brahma Randra in your body. One billion (100,00,000) mantras are trying to describe this face of Śiva. Ākāśa (Ether) Tattva. The individual is from very less to not at all receptive to social structures. Possesses excellent qualities of controlling mortal and divine beings with ease. The individual has reduced his ego to ashes signifying absolute love for the universe and has been freed from cosmic law.

Meditation.

Depending on your Rāśi, Lagna, Daśā, Antardaśā, Janma Nakṣatra, whichever is strong you should meditate that face of Śiva.

For example, Aghora represents South. If your resultant is south( i.e., rāśi, lagna occupies south direction of Sun in your horoscope), you can mediate Southern face of Śiva i.e., Aghora. Depending on the problematic area of life, Kendra, Koṇa, Duṣṭhāna, Trikoṇa, Āpoklima - you should meditate that face of Śiva to get some relief in that problematic area.

For example, with somebody having Taurus (Vṛṣabha)rāśi as lagna, having Venus is in 11th bhāva i.e., Pisces (Mīna). Running Daśā is Guru Daśā and let’s say his Antardaśā is Saturn.

Now, let’s say in this example, Venus is strong in Pisces, but he is running Guru Daśā, If Guru is present in Dhanus, then we consider him as weak because he is occupying 8th House. Since Guru is present in north east corner of the horoscope he should start with Northern face of Śiva mantra.