If you realize the extra Rak`ah during prayer, sit down immediately and complete the Tashahhud. Then, perform Sujud As-Sahw after the Salam. If you realize after finishing prayer, perform Sujud As-Sahw after the Salam to correct the mistake.

If the worshipper did only one prostration for forgetfulness out of ignorance, he does not have to do anything, and his prayer and the prayer of one who prayed behind him is valid.

If there are two reasons for Sujud as-Sahw, it is sufficient for him to do one prostration for forgetfulness, and what appears most likely is that it should be before the salam.