The instructions for performing sajdat al-sahw are as follows:
one should stipulate the intention for sajdat al-sahw immediately after the salaam of prayer, and place the forehead on an item on which sujud is valid. Obligatory precaution dictates that the other body parts for sujud must also be placed on the ground.
In this position he should recite the following dhikr:
بِسْمِ اللّٰهِ وَبِاللّٰهِ
Bismillaahi wa billaah.
اَلسَّلامُ عَلَيْكَ اَيُّها ٱلنَّبِىُّ وَ رَحْمَةُ اللهِ وَ بَرَكاتُه
Assalaamu 'alayka ayyuhan nabiyyu wa rahmatullaahi wa barakaatuh.
He should then sit, and proceed to sujud again, reciting the aforementioned dhikr. After the second sujud, he should sit and recite the tashahhud and salaam. Obligatory precaution dictates that the tashahhud should be recited in the normal manner.
For salam, he should say:
اَلسَّلامُ عَلَيْكُمْ
Assalaamu Alaykum.