There's probably nothing you need to do mechanically different to play a heretic, as your heresy would probably be based off your disagreements with the churches and religious institutions in the campaign setting that are creating and regulating the rules for whatever god it is they are proselytizing for. You wouldn't be going against your god, but rather the organization that is trying to run how that god is meant to be worshipped in the world.
Thus you probably would want to get creative alongside your DM and once you've chosen which god your character is a cleric for... begin working together to come up with what exactly the god's church and worshipping organization does-- how they require and ask their followers to show their faith, what are the tenets of the faith and what one must do in everyday life to exemplify it, what are the "rules" you must follow to be considered an honorable and faithful member of the church, etc. Then once the earth-bound "religious organization" has been created... you can decide what are the parts or rules of that organization that your character breaks with-- and thus be deemed a "heretic".
In the Eberron campaign setting there's a religion called the Church of the Silver Flame that has a very well-defined doctrine, and a long history of what they believed in and how the religion has evolved over the centuries. So now you can have characters that follow exceedingly orthodox doctrine, or are very progressive in their beliefs of what the Silver Flame believes in and works, and everywhere else in between (including the ways a Church member might have broken from the tenets of the Church.) If you happen to own the 3.5 Eberron Campaign Setting (or even better, Faiths of Eberron) you might be able to find all kinds of cool ideas to mine from it for how the church of whatever your god is is designed and run... and thus be able to make a compelling heretic out of it too.
Thus you probably would want to get creative alongside your DM and once you've chosen which god your character is a cleric for... begin working together to come up with what exactly the god's church and worshipping organization does-- how they require and ask their followers to show their faith, what are the tenets of the faith and what one must do in everyday life to exemplify it, what are the "rules" you must follow to be considered an honorable and faithful member of the church, etc. Then once the earth-bound "religious organization" has been created... you can decide what are the parts or rules of that organization that your character breaks with-- and thus be deemed a "heretic".
In the Eberron campaign setting there's a religion called the Church of the Silver Flame that has a very well-defined doctrine, and a long history of what they believed in and how the religion has evolved over the centuries. So now you can have characters that follow exceedingly orthodox doctrine, or are very progressive in their beliefs of what the Silver Flame believes in and works, and everywhere else in between (including the ways a Church member might have broken from the tenets of the Church.) If you happen to own the 3.5 Eberron Campaign Setting (or even better, Faiths of Eberron) you might be able to find all kinds of cool ideas to mine from it for how the church of whatever your god is is designed and run... and thus be able to make a compelling heretic out of it too.
