If you do something extra in the prayer by mistake, you should offer Sujud As-Sahw after the Salam, and if you omit something, then you should do Sujud As-Sahw before the Salam.

If you join the prayer late, you should follow the Imam if he does Sujud As-Sahw before the Salam, but you should not follow him if he does it after the Salam because the one who joins the prayer late cannot say the Salam with the Imam.

There is no specific Dhikr to be said in the two prostrations of forgetfulness (Sujud As-sahw). You can say what you say in the prostration in prayer such as (Subhan Rabbiy Al-a`la). Between the two prostrations, you can say ‘Rabbiy ighfir li’.

Sujud as-Sahw in prayer is prescribed for three reasons: 1- Doing something extra, 2- Omitting something, 3- Being uncertain. Sujud as-sawh should be done before the salam if you have omitted one of the obligatory parts or you are not sure how many rak`ahs you have done. Sujud as-sahw should be done after the salam if you have added something extra or you are not certain but one of the two choices seems more likely to be the case.