When to Offer Sujud As-Sahw
Question

I would like to ask about how the prostration of forgetfulness is to be done in the case of omitting or adding something in the prayer. If the prostration of forgetfulness comes after the Taslim, should the worshipper repeat the Tashahhud or not? 
Should he say Subhana Rabbiyal-A`la (Glory be to my Lord most High) during the prostration of forgetfulness? Or are there other Dhikrs that can be said in the prostration of forgetfulness? 
If the worshipper forgets to say the first Tashahhud, does he have to do the prostration of forgetfulness or not?
Praise be to Allah.When to offer Sujud As-Sahw
With regard to when the (Sujud As-Sahw) prostration of forgetfulness should be done, before or after the Salam, there is a great difference of opinion among the scholars. The most correct view is that if a person does something extra in the prayer by mistake, the prostration must be done after the Salam, and if he omits something then he should do the prostration before the Salam.
If he is not sure, then it is subject to further discussion. If one thing seems more likely to him than the other, then he should do the prostration after the Salam, and if neither seems more likely, then he should do the prostration before the Salam. 
What to do if you miss the first Tashahhud
It says in Fatawa Al-Lajnah Ad-Da’imah (8/7):  
The first Tashahhud in the prayer is one of the obligatory parts of prayer, according to the more correct of the two scholarly views, because the Prophet (peace and blessings of Allah be upon him) used to do it and he said: “Pray as you have seen me praying.” If he omitted it by mistake, he should prostrate because of forgetting, and if he omitted it deliberately, then his prayer is invalid. The one who omitted it by mistake can make up for it by doing the prostration of forgetfulness before the Salam.”
Is there Tashahhud after Sujud As-Sahw?
It is not prescribed to repeat the Tashahhud after the prostration of forgetfulness , whether it comes before or after the Salam. 
What to say in Sujud As-Sahw
The prostration of forgetfulness is to be done like the prostration in prayer, so one should prostrate on seven bones as in the prostration of prayer, and remember Allah by reciting the well-known Dhikr (Subhana Rabbiyal-A’la (Glory be to my Lord most High)) and saying between the two prostrations Rabb ighfir li, Rabb ighfir li  (Lord forgive me, Lord forgive me). There is no special Dhikr for the prostration of forgetfulness. This is what has been stated by the scholars. 
Al-Mardawi (may Allah have mercy on him) said in Al-Insaf (2/159): 
“The prostration of forgetfulness , what is said during it and after rising from it are the same as the prostration in prayer.”
Al-Ramli (may Allah have mercy on him) said in Nihayat Al-Muhtaj (2/88): 
“The manner in which it (i.e., the prostration of forgetfulness) is done is the same as the prostration in prayer, with regard to its obligatory duties and recommended parts, such as placing the forehead on the ground, being at ease in the posture, and Iftirash (sitting on the left thigh with the right foot upwards and its toes pointed towards the qiblah) when sitting in between them.”
Some jurists regard it as recommended to say Subhana man la yas-hu wa la yanam (Glory be to the One Who does not forget or sleep) during the prostration of forgetfulness but there is no evidence for this. What is prescribed is to limit oneself to the Dhikr that is said when prostrating during prayer, and not to recite any other Dhikr . 
And Allah knows best.