If your company were a person, what would it sound like?
Like a person, a company has a communication style that makes up its personality. It's the "sound" that your customers recognize in your messaging. For example, Brooklyn Roasting Company is bright and positive - they use colloquial words like "fun," "tasty," and 'zesty.'
MailChimp is whimsical and conversational. Their audience feels as if they are talking to a friend. On the other hand, Slack is authoritative, powerful, and serious.
To set up a brand voice guide, collect your anchor documents. These are your mission, vision, values, the problems you solve for your client, and the "why" for your business. The phrases and keywords you pull from these documents will be a clue to how you use language and what things are important to your company.
A second great place to find clues about your style and tone is your audience. Do some social listening to see how your audience uses language. Audit your content to see what performs well and why.
Here is a simplified example of some of the elements that make up my brand voice:
Brand Voice is a critical aspect of your marketing. It defines your company, displays what you stand for, and builds credibility through cohesion. Successful brands do not guess at this! They create and recreate this with intention.