SB 9.14.44-45

sthālī-sthānaṁ gato ’śvatthaṁ
 śamī-garbhaṁ vilakṣya saḥ
tena dve araṇī kṛtvā
 urvaśī-loka-kāmyayā
urvaśīṁ mantrato dhyāyann
 adharāraṇim uttarām
ātmānam ubhayor madhye
 yat tat prajananaṁ prabhuḥ
Synonyms: 
sthālī-sthānam — the place where Agnisthālī was left; gataḥ — going there; aśvattham — an aśvattha tree; śamī-garbham — produced from the womb of the śamī tree; vilakṣya — seeing; saḥ — he, Purūravā; tena — from that; dve — two; araṇī — pieces of wood required for igniting a fire for sacrifice; kṛtvā — making; urvaśī-loka-kāmyayā — desiring to go to the planet where Urvaśī was present; urvaśīm — Urvaśī; mantrataḥ — by chanting the required mantra; dhyāyan — meditating upon; adhara — lower; araṇim — araṇi wood; uttarām — and the upper one; ātmānam — himself; ubhayoḥ madhye — in between the two; yat tat — that which (he meditated upon); prajananam — as a son; prabhuḥ — the King.
Translation: 
When the process of fruitive yajña became manifest within his heart, King Purūravā went to the same spot where he had left Agnisthālī. There he saw that from the womb of a śamī tree, an aśvattha tree had grown. He then took a piece of wood from that tree and made it into two araṇis. Desiring to go to the planet where Urvaśī resided, he chanted mantras, meditating upon the lower araṇi as Urvaśī, the upper one as himself, and the piece of wood between them as his son. In this way he began to ignite a fire.
Purport: 

The Vedic fire for performing yajña was not ignited with ordinary matches or similar devices. Rather, the Vedic sacrificial fire was ignited by the araṇis, or two sacred pieces of wood, which produced fire by friction with a third. Such a fire is necessary for the performance of yajña. If successful, a yajña will fulfill the desire of its performer. Thus Purūravā took advantage of the process of yajña to fulfill his lusty desires. He thought of the lower araṇi as Urvaśī, the upper one as himself, and the middle one as his son. A relevant Vedic mantra quoted herein by Viśvanātha Cakravartī Ṭhākura is śamī-garbhād agniṁ mantha. A similar mantra is urvaśyām urasi purūravāḥ. Purūravā wanted to have children continuously by the womb of Urvaśī. His only ambition was to have sex life with Urvaśī and thereby get a son. In other words, he had so much lust in his heart that even while performing yajña he thought of Urvaśī, instead of thinking of the master of yajña, Yajñeśvara, Lord Viṣṇu.