Gobhilagrihyasutra 1.2.2

1.2.2

dakṣiṇaṃ bāhum uddhṛtya śiro 'vadhāya savye 'm̐se pratiṣṭhāpayati dakṣiṇaṃ kakṣam anavavalambaṃ bhavaty, evaṃ yajñopavītī bhavati;

Raising his right arm, putting the head into (the upavîta), he suspends (the cord) over his left shoulder, so that it hangs down on his right side: thus he becomes yagñopavîtin.