Unlike the Gods Above, the worship to the Gods Below is personal, not codified. How and when you want to pray is is not mandated by anyone. To get an answer to a prayer, you need to give up something of value. This could be the spilling of blood, be it your own or that of others.
It is unknown if all who are devote followers recive a similar reward, but named who have served the Gods Below well are offered a final gift as they die. This could be something as simple as an answer, to casting a great curse.