The gopīs first executed the process of ācamana, drinking a sip of water from the right hand. They purified their bodies and hands with the nyāsa-mantra and then applied the same mantra upon the body of the child.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
गोप्यः संस्पृष्टसलिला अङ्गेषु करयोः पृथक् । न्यस्यात्मन्यथ बालस्य बीजन्यासमकुर्वत
gopyaḥ saṃspṛṣṭasalilā aṅgeṣu karayoḥ pṛthak | nyasyātmanyatha bālasya bījanyāsamakurvata
SB 10.6.21