There are different method of sajda-e-sahw in different books . what is exact method of it and when it is necessary?
If one fears that one has forgotten or omitted or made an unintentional mistake in ones prayer due to forgetfulness, regardless of whether the prayer is obligatory or supererogatory, one should perform the sajdah-sahoo or prostration of forgetfulness; it is expected that the Lord Most Gracious will accept their worship of prayer and will not reduce their reward in the least.
There are two ways prescribed in the authentic narrations on how the Messenger of Allah (saws) performed the Sajdah-Sahoo and both are acceptable. Abu Sa'id al-Khudri reported that the Messenger of Allah (saws) did two sujoods before the tasleem, and in the narration of Dhul-Yadain narrated by Abu Hurairah, it is recorded that the Prophet (saws) made the two prostrations after the tasleem.
The way to perform the Sajdah-Sahoo (or Sajdahs of forgetfulness) is to go into sajdah and glorify Allah, then sit and make supplication to Allah, then again go into sajdah and glorify Allah, and then perform the tasleems on either side.
This whole set of actions related above is called sajdah-sahoo and is to be done only once at the end of the prayer, if the person has made an unintentional mistake/s in their prayer.
Sajdah-Sahoo 1 (before the tasleem):
If one realizes one has made an error in prayer during the prayer, one should complete the prayer to the best of his conviction and recite the tashahud; just before the tasleems of either side at the end of the tashahud, one should make the two sajdahs for sajdah-sahoo and then perform the tasleems on either side; thus reciting the tashahud only once in their prayer.
Sajdah-Sahoo 2 (after the tasleem):
If one realizes that one has made an error in prayer after one has already completed ones prayer and performed the tasleems; then immediately after realization of the error, one should complete the rakahs they had forgotten in prayer, recite the final tashahud again, make the two sajdahs for sajdah-sahoo, and then again perform the tasleems on either side; thus reciting the tashahud twice in their prayers