Anna Tong has a 10 Minute Recipe for Establishing Brand Tone of Voice

If 10 minutes is too long, Alex Honeysett has a 4 Steps to Finding Your Brand’s Voice

Here is a list of other sites and examples of brand voice and tone guides:
MailChimp’s Voice and Tone
Oxford University style guide
Buffer’s voice & tone guide
Steven Pinker’s The Sense of Style
George Orwell’s Five Rules for Effective Writing
David Ogilvy’s 10 Tips on Writing

Good luck!