When an author is creating a world, it can’t have Christians, Muslims, Jews or Buddhists; at least, not named as such. That’s why many fantasy novels have different styles of religions. Some blend the various earth religions into one, or they don’t refer to it at all. That’s how I handled it in The Accidental Colony. I suppose I could have them praying to God or Allah, or whomever Buddhists pray to, for help, but I chose to have them look within themselves to find whatever skills they might need to survive.
Ignoring religion is ok, if your story allows it. Sometimes it doesn’t. Perhaps this is a weakness in my story, but God doesn’t help my hapless scientists, they help themselves, so I can safely leave religion out of the mix. This is good because I don’t know that much about religion other than Catholicism, the religion I was raised in. this is a weakness in me. I can research the other religions but books and websites can leave a lot out, and that would be glaring to practitioners of that particular faith. I find it easier to ignore it. Still, there may be times when an author needs to create a religion. Some fantasy novels need religion to move the story along. Maybe the hero is a monk or nun who needs to battle agnostics to reach his or her story goal. Religion has been the basis of war on Earth; it can be so in a fantasy world as well and probably has. The basic plot of good vs. evil has religious overtones.
When an author absolutely needs a religion, it needs to create a Supreme Being. That Supreme Being will need priests and priestesses. A good way to start is to look at the rituals of ancient times, be it Greek, Roman, Jewish, Muslim or Early Christian. They all had the basic rule of sacrifice. Archaeologists believe that human sacrifice was relatively rare in the Jewish, Christian and Muslim cultures. I’m no expert but I think it might have been relatively rare in the European religions as well. The New World religions practiced it a lot and that could have been the case in ancient Europe as well. It would depend on the religion. Anyway, the method of sacrifice was as important as the sacrificial creature, human or animal, and authors need to make sure that they understand the rituals that they create for their fantasy worlds. That’s not to say that sacrifices have to be described in gory detail in the work, it’s only important that the author know how it was done so that a broad sketch won’t go awry. Nothing pops a reader out of a story like inconsistency and that is what I am talking about here. A sacrifice performed at the beginning of a novel can be detailed, but the sacrifice at the end doesn’t. However, the sacrifice at the end needs to resemble the sacrifice at the beginning. In all things related to world building, consistency is the key.
Prayers should have some formulaic feel to them, but this is not truly necessary. Many people pray in odd moments of stress, and I am sure that they are not spouting a formulaic prayer. These prayers are more in the shape of "Dear God, let me survive this," or "Dear God, please get me out of this." This is both acceptable and realistic, depending on the peril the character is facing. That’s not to say that formal prayers are never to be used, that’s not true. If an author needs to show a group ritual, formulaic prayers are the order of the day.
In the end, the religion chosen can be modeled on a current or archaic Earth religion, or it can be ignored completely or an author can choose to create a completely new religion out of its head. It’s really up to the individual author.