Some ideas and questions on how I've done religion in my low-magic sword & sorcery version of Pathfinder's world Golarion:
1) Is the god as it is presented in the book necessary and interesting? If not, discard or change it.
2) What kind of social role does the religion have?
E.g. in my campaign Pharasma is an cryptic version of Catholicism. They have somber rites, gothic cathedrals, dark robes and tall hats, and their role is to help people with life's transition phases (birth and death). They often upkeep and guard the cemeteries, and keep the dead where they belong. As an organization they're old and hierarchical with formal education.
In Cheliax, the church of Thasaidon (instead of Asmodeus; inspired by Clark Ashton Smith's stories) is the social glue of the nation. Think of the Roman state religions. People do the rites but don't really believe in them, and are free to worship any other deities (except the previous state religion). Any threat to its sovereignty is a threat to civilized society and the rightful rule of the queen. Inside the church it's a mystery cult with very few adherents. Thasaidon is the lord of the seven hells and is thematically tied to Thassilonian seven virtues/sins. It's dark and intimidating, but ultimately indifferent toward the average subject of the Empire.
Desna is venerated by the nomadic Varisians. Their beliefs are passed mostly orally, and are intermixed with folklore and superstitions. She is said to protect travelers and sometimes send prophetic dreams. The favored ones often have deformities or unusual features. There no priests but soothsayers and Varisian witches are thought to best represent Desna's wishes.
3) Fewer religions is better, especially if you make them interesting. Tell stories through imagery, imply things through rituals and legends, let players imagine what you leave unsaid.
4) Limit the religions geographically, have different religions make conflicting statements of the nature of reality. Have sects and different forms of religions (mystery cults, evangelistic, secularized old institutions etc.).
Every man worships Thasaidon in his heart and his sovereignty is indisputable in Cheliax. In the south, in Sarenrae's (Sufist) theology, Thasaidon is the brother of Ahriman the beast who is trapped in hell, and he is the one holding the keys of the prison.
5) Think about the 'face' of the religions. What do priests and rituals look like? What do the fanatics look like (self-castration, child preachers, anything crazy from real history)? Do they have a clear and knowable goal that is present in every day life (Razmirans gather wealth)? Or are they just interwoven to the life of average people (farmers' gods)?
6) Who or what comes in conflict with the religion and its dogmas?
I like to make adversary religions for every religion I create. For Pharasma who wants to keep the dead dead, Urgathoa represent victory over death. For Desna who wants to protect travelers, Lamashtu represents the lusty untamed dangerous wild places and creatures.
7) Invent a couple of major tenets, and a couple of major sins for each religions.
A Pharasman male priest's job is to bless the dead and see that people are given a proper burial. They won't desecrate the dead, or dabble in anything sinister that has something to do with the dead. A strict sect might persecute people wanting to make autopsies, or be merciless towards those who even touch the dead without proper authority.
That's all I can think of off the top of my head.