A style guide or a style sheet is an important editing tool. This is a document that keeps track of all the editing and writing decisions that an author and editor team make.
What does it look like?
In my experience, it is a Microsoft Word document. However, it is typically a collection of decisions, grammar rules, lore, timelines, etc. that relate to a standalone novel or series. You can organize it however you like but there are templates provided from the EFA classes and sometimes fellow editors (aka edibuddies). In the editing community, edibuddies may share organizational tips or sections that they include for different genres that they work with.
For instance, I break down multi-world lore if the characters are planet or dimension hopping and include excerpts in the style guide pulled from the manuscript to keep exact wording consistent across series or within a book.
What does it contain?
Style notes (capitalization, hyphenation, numbers, punctuation, acronyms or abbreviations, and miscellaneous preferences ), an alphabetized word list, proper names, locations, and lore notes.
As said before the genre may influence additional sections but these are the big ones. I often include a timeline and note when character deaths happen as well as who died to make sure they don't reappear some time later without an explanation.
What do I do with it?
From the writer's perspective, this is a handy document that tells you exactly what your editor is seeing and reading. This can help ensure consistency in your vision from what you want to show your readers and what they are actually reading. Your editor can help you close the gap (if there is one) and bring your dream to life. If this book is part of a series then when you write the next one you already have a reference of past decisions to guide you and expand upon the world building you have already done.