SB 6.8.7
kara-nyasam tatah kuryad
dvadasaksara-vidyaya
pranavadi-ya-karantam
anguly-angustha-parvasu
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Then one should chant the mantra composed of twelve syllables [om namo bhagavate vasudevaya]. Preceding each syllable by the omkara, one should place the syllables of the mantra on the tips of his fingers, beginning with the index finger of the right hand and concluding with the index finger of the left. The four remaining syllables should be placed on the joints of the thumbs.