the method of self purification is given in detail in some ancient texts as follows...
त्रिः प्राश्नीयादपो यत्तु प्रीतास्तेनास्य देवताः
ब्रह्मा विष्णुश्च रुद्रश्च भवन्तीत्यनुशुश्रुमः
गङ्गा च यमुना चैव प्रीयते परिमार्ज्जनात्
संस्पृष्टयोर् लोचनयोः प्रीयते शशिभास्करौ।
ना सत्यसंज्ञौ प्रीयेते स्पृष्टे नासापुतद्वये
कर्ण्णयोर्स्पृष्टयोस्तद्वत् प्रीयते चानिलोनलौ
अम्सयोर्स्पर्शनादेव प्रीयते तु प्रजापतिः
संस्पृष्टे हृदये चास्य प्रीयते सर्वदेवताः
मूर्ध्नि संस्पर्शनादेव प्रीतस्तु पुरुषो भवेत्
य एवं ब्राह्मणो नित्यं उपस्पर्शनमाचरेत्
ब्रह्मादिस्तम्बपर्यन्तं जगत् सः परितर्पयेत्
अङ्गुष्टोऽग्निरिति ख्यातः प्रोक्ता वायु प्रदेशिनी
अनामिका तथ सूर्यः कनिष्ठा मघवाः स्मृता
प्रजापतिर्मद्ध्यमा तु ज्ञेया शुद्धिमभीप्स्यता
triḥ prāśnīyādapo yattu prītāstenāsya devatāḥ
brahmā viṣṇuśca rudraśca bhavantītyanuśuśrumaḥ
gaṅgā ca yamunā caiva prīyate parimārjjanāt
saṁspṛṣṭayor locanayoḥ prīyate śaśibhāskarau|
nā satyasaṁjñau prīyete spṛṣṭe nāsāputadvaye
karṇṇayorspṛṣṭayostadvat prīyate cānilonalau
amsayorsparśanādeva prīyate tu prajāpatiḥ
saṁspṛṣṭe hṛdaye cāsya prīyate sarvadevatāḥ
mūrdhni saṁsparśanādeva prītastu puruṣo bhavet
ya evaṁ brāhmaṇo nityaṁ upasparśanamācaret
brahmādistambaparyantaṁ jagat saḥ paritarpayet
aṅguṣṭo'gniriti khyātaḥ proktā vāyu pradeśinī
anāmikā tatha sūryaḥ kaniṣṭhā maghavāḥ smṛtā
prajāpatirmaddhyamā tu jñeyā śuddhimabhīpsyatā
Touching of the various parts of the body, and partaking in holy water during aachamanam are explained in detail by the lawgiver Haartia.
By drinking water three times during aachamanam, the lords Brahma, Vishnu and Siva are pleased. When we rinse our face with water, the holy rivers Ganga and Yamuna are pleased. When we touch the two eyes, the sun and the moon are pleased. The touching of the two nostrils propitiate the divine doctors, the Ashwins. When we touch the two ears Vayu and Agni are pleased. When we touch our shoulders, Prajapati is pleased. When we touch our heart, all the deities are pleased. And when we finally touch our head the Supreme Lord Parabrahmam is pleased. When a Brahmin does such an aachamanm, he gets the benediction of the whole universe right from the Brhmaloka. (It is believed that evil spirits like bhuutapretapishacha cannot attack a person who has done aachamanam like this.)
While doing self purification one has to understand that agni or fire is residing in his right thumb, vaayu is present in his index finger, surya is resident in his ring-finger, Indra in his small finger, and prajaapati in his middle finger.
We clean the face with the thumb, touch the eyes with the ring finger, nostrils with the index finger, ears with the little finger, shoulder with the middle finger, heart and head with all the five fingers. Some great rishis are of the view that since there is the presence of agni in the thumb, we should use thumb along with the respective fingers when we touch the various organs. We use the name Kesava and Narayana for cleaning the face, Madhava and Govinda for the eyes, Vishnu and Madhusuudhana for the nostrils, Thrivikrama and Vamana for the ears, Sreedhara and Hrishikesha for the shoulder, Padmanabha for the heart and Damodhara for the head. The touching is done by the right hand and start from the right. This , I hope will explain the ritual behind touching of the variout parts of the body and self purification according to scriptures.