Karma Vipaka Samhita 32.21

कुण्डे कोणत्रये चैव होमं वै कारयेत्ततः ॥

Then he should have a homa (fire offering) done in a pit with three corners. The homa and the rest should be a tenth of the japa (repeating a mantra).

ततो वै प्रतिमां कुर्याद्वैश्यस्यैव विधानतः ॥

Then he should make an image of the merchant by rule. It is of twelve pala (a unit of weight) of gold, with care.

पूजयेत्पूर्वजैर्मन्त्रैस्ततो विप्राय दापयेत् ॥

He should worship it with the mantras (sacred words) told before, and give it to a brahmin. When this is done, no doubt, a son is born, not otherwise.