The tashahhud should be recited quietly with concentration, and your eyesight should be fixed on your index finger which is pointing towards the Qiblah. The minimum that needs to be said during a sitting of the middle tashahhud is the following:
Transliteration: at-taḥiyyaatu lillaahi waṣ-ṣalaawaatu waṭ-ṭayyibaatu, as-salaamu ‛alayka ayyuhan-nabiyyu wa raḥmatullaahi wa barakaatuh, as-salaamu ‛alaynaa wa ‛alaa ‛ibaad-illaahiṣ-ṣaaliḥeen, ash-hadu an laa ilaaha ill-allaahu, wa ash-hadu anna muḥammad-an ‛abduhu wa rasooluh.
Translation: Salutations are for Allah, All acts of worship and good deeds are for Him. Peace, mercy and blessings of Allah be upon you O Prophet. Peace be upon us and upon all of Allah's righteous slaves. I bear witness that none has the right to be worshipped except Allah and I bear witness that Muhammad is His slave and Messenger.
Note: Some of the major companions of the Prophet Muhammad (may the peace and blessings of Allah be upon him), such as Ibn Mas'ood and Aa'ishah would say "as-salaamu ‛alan-nabiyyi", meaning: "peace be on the prophet" instead of "peace be upon you O Prophet”, after the death of the Prophet Muhammad.
The proof for the wording of the tashahhud is found in the following narration:
Ibn Mas'ood (may Allah be pleased with him) narrated:
"Allah's Messenger (may the peace and blessings of Allah be upon him) taught me the tashahhud, like he would teach me a chapter of the Qur'an, while my hand was between his hands. [It was:] Salutations are for Allah, All acts of worship and good deeds are for Him. Peace, mercy and blessings of Allah be upon you O Prophet. Peace be upon us and upon all of Allah's righteous slaves. I bear witness that none has the right to be worshipped except Allah and I bear witness that Muhammad is His slave and Messenger. [We used to recite this in the prayer] during the lifetime of the Prophet (may the peace and blessings of Allah be upon him), but when he had died, we used to say, 'Peace be on the Prophet.'" [al-Bukhārī: 6265]
In addition to this, you should also send blessings upon the Prophet (known in Arabic as aṣ-ṣalātu ʿalan-nabī, and in some other languages as durood). It should be said regardless of whether the person is in the middle tashahhud or the final tashahhud.
This supplication is said as follows:
Transliteration: allaahumma ṣalli ‛alaa muḥammadin wa ‛alaa aali muḥammadin kamaa ṣallayta ‛alaa ibraaheema wa ‛alaa aali ibraaheema, innaka ḥameedun majeed, allaahumma baarik ‛alaa muḥammadin wa ‛alaa aali muḥammadin kamaa baarakta ‛alaa ibraaheema wa ‛alaa aali ibraaheema, innaka ḥameedun majeed
Translation: O Allah, send prayers upon Muhammad and the followers of Muhammad, just as You sent prayers upon Ibrāheem and upon the followers of Ibrāheem. Indeed, You are full of praise and majesty. O Allah, send blessings upon Muhammad and the followers of Muhammad, just as You sent blessings upon Ibrāheem and upon the followers of Ibrāheem. Indeed, You are full of praise and majesty.
If you are only performing a prayer which consists of two rak'ah (units of prayer), such as the Fajr prayer, then skip the next section and go to the final tashahhud.
If you are performing a prayer which is longer than 2 units, then continue to the third and fourth rak'ah (units of prayer).