Is there a specific rule on how you should refer to someone of unknown gender in French? For example, if you were writing a guide on how to do something and you wanted to indicate that the reader should do something, which of the below (or something else?) would it be best to write?

Il doit faire...
Elle doit faire...
On doit faire...
Il faut faire...

This question also applies if someone's gender is in dispute; how would you refer to that person?