After completing the final prostration in the last rak’ah (unit of prayer), whether that is the second, third or fourth rak'ah, you should begin the final tashahhud.
The final tashahhud follows the exact same format as the middle tashahhud, except that it has extra parts at the end to finish the prayer.
It is mentioned in various narrations that the Prophet Muhammad (may the peace and blessings of Allah be upon him) would sit differently in the second tashahhud, for those prayers in which there are two.
This sitting position is known in Arabic as tawarruk, and in this position, the upper left thigh is resting directly upon the ground, instead of resting upon the lower leg, and both feet protrude out of the right side. The left foot comes out from under the right thigh and shin, and the right foot is propped upright. In this position, the left hand leans heavily upon his left knee in this position.
Abū Ḥumayd as-Sā'idī (may Allah be pleased with him) narrated:
"When the Prophet (may the peace and blessings of Allah be upon him) sat at the end of two rak'ah, he sat on the sole of his left foot and raised the right, and after the fourth he placed his left hip on the ground and spread out both his feet to one side." [Abū Dāwood: 730]
After you have read the tashahhud supplication and the supplication of sending blessings upon the Prophet (may the peace and blessings of Allah be upon him), you should supplicate to Allah with the following supplication:
اللَّهُمَّ إِنِّي أَعُوذُ بِكَ مِنْ عَذَابِ الْقَبْرِ
وَمِنْ عَذَابِ جَهَنَّمَ وَمِنْ فِتْـنَةِ الْمَحْيَا وَالْمَمَاتِ
،وَمِنْ شَرِّ فِتْـنَةِ الْمَسِيحِ الدَّجَّالِ
Transliteration: allaahumma innee a‛oodhu bika min ‛adhaabil-qabri, wa min ‛adhaabi jahannam,
wa min fitnatil-maḥyaa wal-mamaat, wa min sharri fitnatil-maseeḥ-id-dajjaal
Translation: O Allah, I seek refuge with You from the torment of the grave,
from the torment of the Fire, from the trials and tribulations of life and death
and from the evil affliction of the Antichrist.
After completing this supplication, there are many other optional supplications to choose from, which can be found in the following links:
http://duas.com/dua/200/dua-after-the-last-tashahhud-and-before-salaam-2
http://duas.com/dua/201/dua-after-the-last-tashahhud-and-before-salaam-3
http://duas.com/dua/202/dua-after-the-last-tashahhud-and-before-salaam-4
http://duas.com/dua/203/dua-after-the-last-tashahhud-and-before-salaam-5
http://duas.com/dua/204/dua-after-the-last-tashahhud-and-before-salaam-6
http://duas.com/dua/205/dua-after-the-last-tashahhud-and-before-salaam-7
http://duas.com/dua/206/dua-after-the-last-tashahhud-and-before-salaam-8
http://duas.com/dua/207/dua-after-the-last-tashahhud-and-before-salaam-9
http://duas.com/dua/208/dua-after-the-last-tashahhud-and-before-salaam-10
http://duas.com/dua/209/dua-after-the-last-tashahhud-and-before-salaam-11