1 After Maghrib, to pray six Rakats is Mustahhab and they are known as Salaatul Awaabeen. It is better to pray them in two, two Rakats [Durr-e-Mukhtar, Radd-ul-Mohtar].
2 the Mustahhab of Dhuhr, ‘Asr and ‘Isha include the Sunnah-e-Maukida. For example if you pray four Rakats after Dhuhr Fardh then both the Sunnahs and the Mustahhab will be fulfilled and alternatively you can pray them both with one Salaam. Meaning perform Salaam after four Rakats and only the intention of Sunnah is sufficient, there is no need to define the Maukida and Mustahhab separately as they both will be fulfilled [Fatahul Qadir, Bahar].
3 Qiraayat is obligatory in all the Rakats of Nafl and Sunnah.
4 To intentionally start a Sunnah or Nafl Salaah will make it Wajib, meaning if you break it then to repeat or perform it’s Qadha is Wajib.
5 Nafl Salaah can be prayed without reason whilst sitting down, however, to pray it standing up carries double reward [Hidaya].
6 If you are praying Nafl sitting down then sit as if you are sat in Qaidah except for when you are praying Qiraayat have your arms folded below the navel just as you have them when stood up [Durr-e-Mukhtar, Radd-ul-Mohtar].
7 The two Rakats Nafl that are prayed after Witr, it is better to pray ‘Iza Zul Jilatul Ardo’ in the first Rakat and ‘Qul Yaa Ayyuhal Kafiroon’ in the second Rakat.