wats the niyaat (intention) for shukranai ke nafal
You make a niyah for farz as you do for vitr or sunnah. The same applies for nafil, just that you incorporate that these nafils are for showing gratitude to Allah.
i.e. 2 rakah shukranay nafil, nafil wasta Allah, facing towards the ka'bah Allah o Akbar.
jazakallah brov'