The remedy for the couples who doesn't have child is mentioned in Chapter 63 - Of Varaha Purana - PutraPrapti Vrata. This vrata is worshiping Lord Vishnu as Shree Krishna.
Here is the entire Vrata with the description of Homa you enquired from varaha Purana.
Agastya Said -
1 : O king now listen to the brief account of the Putra-Prapti Vrata ,
I am going to give you.
2: This Putra-Prapti Vrata (vow for getting son) is to be performed
fasting on the Astami tithi in the dark half of the month of
3: The sankalpa should be made on Saptami day and the worship of vishnu as krishna lying in the Devaki's lap surrounded by the mothers
should be conducted on Astami.
4: On the Astami Day ,early in the morning ,Vishnu should be worshiped
with full concentration of mind.
5: Then Homa should be performed with barley ,black sesamum ghee and curd .Brahmans should be fed and given Dakshina.
6: Then the performer of the Vrata may take food ,first Bilva leaves
and then with oils and other items.
7: Doing in this way the KrishnaPaksha Astami every month ,an
issueless person begets children.
8-10 : It is known that in the ancient times the valiant king surasena
who was issueless ,performed penance in the Himalayas. And Lord
himself told him about this vrata and accordingly he performed it and
got a son named Vasudeva by name who later on performed many
sacrifices and the saintly king surasena gained salvation.
11 :This o king I have told you about Krishnasami (in this vrata) ,at
the end of the year ,a pair of cows should be gifted to Brahmin .
- Such is the Putravrata narrated to you. By performing it one get freed from all sins.