I primarily use the Chicago Manual of Style (CMOS), the Associated Press (AP), the Oxford Style Guide, and my clients’ own style guides to achieve an accurate and polished end product. I also frequently refer to The Best Punctuation Book, Period.
I edit in Microsoft Word with its Track Changes feature to highlight and track all errors I identify and changes or notes I make. It allows clients to review their work and choose to accept or reject my changes.
If the project requires, I will also create a job-specific style sheet that lists selected grammar, punctuation, spelling, etc., choices to ensure continuity on similar projects.