How to Create a Style Guide for Your Application

I've been learning quite a lot from the work of people like Brad Frost and Anna Devenom who've been writing about style guides. And, and defenders listening, I listen to Rachel. She's written a lot of books and it is not easy to write a book. So definitely listen to those words there, defenders. Great input. What's your most invincible UX resource or tool you can recommend to our listeners?