Ask HN: I need to create a Style Guide - where do I start?

Hi HN! I'm a developer/designer approaching the design phase of a very ambitious personal project. I have identified the need for a style guide much like the recent one put out by the BBC redesign. However, I've never created a style guide before so I'm not sure what software I should be using, what the standards are, if any. What's the best place to start?<p>Thanks everyone!