2- Omissions in Salah(rukn/wajib/sunnah)> 2a- omission of Rukn (pillar)
then depends when you remembered the omission of the rukn... so whatever the case maybe in the 'make up' ra'kah one should recite the tashahud in it then followed by a sujood as sahw after the tasleem.
Shaikh Muhammad ibn Saalih al-Uthaimeen rahimahullah has a treatise named "Sajdatus-Sahw" .One may refer this book for more detail about Sujood as sahw.
And Allah knows best..
Abu Bakr Al-Siddiq Radhiyallahu anhu said: I will not leave anything Allah's Messenger Sallahu alihi wa ssallam did, except that I will also do it; for I fear that if I were to leave any of his commands and ways I would deviate.(Al-Bukhari, Al-Sahih 2:386