Pray the first two Raka'at just as you would in any other Salaah and on completing reciting Tashahhud in Qa'dah Oola immediately stand up in Qiyaam for the third Raka'ah 

In the third raka'ah:

1. Recite Tasmiyyah, Surah al-Faithah, Tasmiyyah and another Surah/Ayaat.
2. Perform the Takbeer al-Qunoot by saying the Takbeer and lifting the hands (to earlobes for men; shoulders for women) and refasten them in normal position in Qiyaam.
3. Recite Dua al-Qunoot (see below).
4. Complete the rest of the Salaah  as you would ordinarily do so by performing Rukoo', Qawmah, Sujood (incl. Jalsah), Qa'dah Akeerah and Khurooj bi Sun'ihi

Dua al-Qunoot:

اَللّٰهُمَّ   اِنَّا نَسْتَعِیْنُكَ وَ نَسْتَغْفِرُكَ وَ نُؤْمِنُ بِكَ وَ نَتَوَكَّلُ عَلَیْكَ وَ نُثْنِیْ عَلَیْكَ الْخَیْرَ وَ نَشْكُرُكَ وَلَا نَكْفُرُكَ وَ نَخْلَعُ وَ نَتْرُكُ مَنْ یَّفْجُرُكَ اَللّٰهُمَّ   اِیَّاكَ نَعْبُدُ وَ لَكَ نُصَلِّیْ وَ نَسْجُدُ وَ اِلَیْكَ نَسْعٰی وَ نَحْفِدُ وَ نَرْجُوْا رَحْمَتَكَ وَ نَخْشٰی عَذَابَكَ اِنَّ عَذَابَكَ بِالْكُفَّارِ مُلْحِقٌ۝

if this is not known, then:

رَبَّنَاۤ  اٰتِنَا فِی الدُّنۡیَا حَسَنَۃً  وَّ فِی الۡاٰخِرَۃِ حَسَنَۃً  وَّ قِنَا عَذَابَ النَّارِ 

or, 3 times:

 اَللّٰهُمَّ اغْفِرْلِیْ