We can decide the details on how and when people worship (whether characters follow this or not). If our story doesn’t need much, keep this simple. It helps to know when a priest or religious character is unavailable because they must worship somewhere at a given time, with other characters aware of this.
For location, they could use a church/mosque or shrine to worship. The former will have priests who can lead prayers. A small shrine is likely to have fewer priests, if any, and the level of formality may be lower, but shrines can be churches by another name, and their size will reflect this. A god of war might want a large, formidable structure, as a god of greed could want something ornate. We can spin this in different ways so just choose something that seems sensible for the deity.
Followers might also use their home or something in the wilderness, like a sacred grove. The latter is more likely for a nature goddess, for example, while it’s a practical matter to worship from home. This can also suggest a time of day, such as morning or night. A more domineering god may be strict while a more benevolent one might not care, but this is also about the religion, and species create these, which means they might be the strict ones. We can decide that people must attend a formal worship at a given interval, such as once a week, with less formal worship expected other times.
When people pray, do they kneel or stand? If on the floor or ground, do they use a mat and what is it made of? Maybe there’s a sacred kind of reed or cloth it’s spun from, or it must be decorated with a symbol or color. Do people use a talisman in their prayers, like the cross or rosary beads? Things used during worship allow an easy way for enemies to defame the god, by defacing what the worshipers use, which can be as simple as stepping on it, if the bottom of the foot is considered unclean?
Some religions require fasting, which can be an interval of our choosing, such as one day a week or a period of sunrise-to-sunset for several weeks, once a year. The timing will coincide with the most holy of periods in the religion. There are advantages to the body, such as increased metabolism and improved concentration, but the reasons for religions to desire fasting is for purification of the body and, by extension, the mind and soul. The goal is often akin to seeking a god’s forgiveness or a similar, humble virtue. Gods that might desire this could be those where purity or devotion seem desirable, and this can extend from food and drink to sex. Specific foods can be forbidden due to a negative association, such as that animal playing a role in a story; if the prophet was searching for his lost flock of some animal when he became a prophet (or when he died), then this can result in not only food from that animal, but fur and other products, being desired or shunned. It’s possible that a god or religion could also insist on certain foods and drinks being consumed in great quantities, such as a week-long feast once a year, and smaller feasts being once a week. Sacrifice can include animal life – and that means the humanoid species, too.
Many religions have a holy text, regardless of form (book, scroll, stone tablets, iPad), but some may be oral. Illiteracy can lead many to depend on priests, which gives them even more power. Religious songs like hymns will exist and if we’d like a character to sing a few lines, we’ll need to compose a portion of it. Some songs might be in a language some characters don’t understand.
Religions can declare days or entire weeks as holy periods. Whether these are recognized by a settlement or sovereign power is another matter to be indicated in our world building files. These times will correspond to significant historical events, such as the day the prophet became one, died, or was born (or reborn). This is one reason we need history. If sacred texts or artifacts were revealed, created, developed, destroyed, or used memorably, each can be associated with a holiday. We may be developing multiple religions and can end up with a holiday every week if we’re not careful. Two opposing religions can clash over a shared holiday.
Religious events can involve specific prayers on a given day(s) and at a holy site that is mobbed by crowds, which can cause problems, from lack of adequate food and shelter to stampedes and accidental death, from lack of adequate food and shelter to stampedes and accidental death. A largescale pilgrimage is likely only once a year. A commemorative event like this is based on a historical one. Leverage the history we’ve invented.