This answer says if anybody missed to perform Sandhyavandanam within prescribed time, Prayaschitta Arghya should be given.
My question is how to give Prayaschitta Arghya?
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This arghyam is given when we miss performing a sandhyavandanam i.e. before the sunrise in the morning, before the sun gets overhead in the evening and before the sunset in the evening. If we perform within the prescribed time, then it is not needed.
We have to do praNaayaama, say sankalpam that we are giving a prayaschitta arghya. Then we have to give the arghya just like we do it before*. This is called Kālātīta prāyaścitta arghyaṃ.
*According to some sampradays, the arghyam offered is thrice at all the parts of the day. Whereas, in some other , it is once in the morning, twice in the noon and thrice in the evening. This change is due to the shakshas and Grihya sutras they follow.
Ajay, you should have learned Sandhya Vandana from your father or acharya or local matam or ashram, soon after upanayanam was done. If you have forgotten the mantras, full details of sandhya vandana here (Sanskrit & Tamil.. also english)
But if none has taught you, do not self-learn.. first learn from elder, then use above as reference to refresh memory.