So now you've chosen a style guide, and you're familiar with it. The trouble is that your guide doesn't cover every single situation that you'll encounter in the particular kind of writing you do. Maybe you disagree with how the style guide handles a particular issue. For instance, perhaps you need to capitalize something that The Chicago Manual of Style doesn't. Or you like everything about AP Style except that you want to use serial commas.
The solution is to customize the style guide for your needs by creating a style sheet. According to The Free Dictionary, a style sheet summarizes "the editorial conventions to be followed in preparing text for publication." A style sheet can serve as a quick reference to highlights from the main style guide along with exceptions to that guide, or it can just focus on the exceptions.
Make your style sheet short and reader-friendly. Don't summarize the style guide. Include the most common situations and give page number references to the main style guide so writers and editors can dive into the details. Bulleted items and checklists work well.
Define and document the exceptions to the style guide rules. Generally, exceptions will crop up in capitalization or how to handle measurements of money, dimensions, and time. Fractions and numbers often result in exceptions, along with how to handle URLs, terms of art, or an organization's traditions. For example, I edited for a military website where the rule was to list the branches of service in order of their establishment, with the oldest first.
Once you've created the style guide, make sure every writer and editor has it. It's a living document, and you'll need a way to gather suggested revisions and update it regularly. The biggest problem with organizational style sheets is that they become obsolete and no one uses them.
Choosing a style guide is only the first step. Once you know the guide well, figure out your exceptions and document them in a style sheet. Give it to everyone who's writing or editing and don't forget about updating.