In the editing I do for businesses and organizations, there is already, most commonly, a style guide. This guide to spelling, capitalization, formatting, and certain grammar and punctuation variations is useful so that all writers and editors for the organization are using the same things. Or an organization, like the School of Nursing at OHSU, will follow the American Psychological Association's style guide, know as APA formatting. But when I edit for individual writers, we generally need to create a style guide, either for that author or for that work. Here are some things you might want to consider as a writer in creating your own guide.
1. Will you use the Oxford/serial comma (the comma before "and" in a series)? The rule now is that this piece of punctuation is optional but should be used consistently (i.e., always or never)?
2. If you're writing fiction, will you include colons and semi-colons? As these are considered formal marks of punctuation, they occur rarely in fiction although on occasion in literary fiction. How formal is the writing you're doing?
3. What is your rule for the use of a comma before the main subject and verb of a sentence? Current usage dictates the use of a comma if the introductory material is a clause (contains a subject and verb) or if the introductory phrase has more than four words (When he left, she was bewildered OR A week ago Friday, he would never have thought that). But there's lots of leeway on this in fiction, as long as you're consistent. What usage of punctuation might make your writing the most reader-friendly?
4. Most of us are good at capitalizing proper nouns and titles. Are there any special words that you want to capitalize? If so, it would be good to have a list.
5. Be sure you create a list of character names, town names, etc., so you can keep the spellings straight.
6. Will you include any character's thoughts? Italics is the most common formatting for such a style.
7. Will you use quotation marks to indicate characters' speech or not? Quote marks is most common but other styles are possible.
While none of these issues need to be decided before you begin writing, keeping track of these decisions will help you in revising and self-editing.