Writing a full set of rules for a multifaceted board game is quite a challenge. Even though I already had a preliminary rules book for the game in word doc it still took me in excess of 9 hours to convert and redo in Photoshop.
Some key things to follow through on for rules, that I've read about, include:
Introduction to the game... Kind of like an Inciting Incident in a book
Path to Vicotry mentioned early on (and clarification at the end)
Game Contents summary... Its good to know what SHOULD be in the box, and how they are labelled
Game Setup... Before describing gameplay the players should have step by step instructions on how to handle every piece/card included with the game (even if it seems simple, it may not be to newer game players)
Gameplay... A breakdown of overall gameplay, as well as step by step on how to get through a complete turn. This also includes an analysis of core game components that will impact gameplay (IE event cards, battles, etc)
Examples... Depending on how complicated things are this could be done outside of the rulebook... still deciding for myself how to do this
Winning the Game ... finish off with how the game is won, included details for ties or complications you might foresee
And that it... but very time intensive and I know I still have to go back and invest more time to fix some typos/spacing and possible gameplay changes after some more playtests. FUN!