According to scholarly authorities the mantrasnāna is as follows: Repeating the mantra “Āpo hi ṣṭhā” etc. water shall be sprinkled over the head for suppressing sins. Repeating the mantra “Yasya Kṣayāya” etc. water shall be sprinkled over the joints in the legs.
ईषत्स्पर्शे च दौः स्वास्थ्ये राजराष्ट्रभयेऽपि च । अत्यागतिकाले च मंत्रस्नानं समाचरेत् २४ ।
It is enough if one performs mantra snāna when one is slightly indisposed, or when there is danger from the king or when there is civil commotion, or when there is no other way or when one is about to undertake a journey.
प्रातः सूर्यानुवाकेन सायमग्न्यनुवाकतः । अपः पीत्वा तथामध्ये पुनः प्रोक्षणमाचरेत् २५ ।
He shall drink by way of Ācamana reciting the mantras from Sūryānuvāka in the morning or from Agni-Anuvāka in the evening and perform the ceremonial sprinkling in the middle.