So, you now know you have at least two styles that you want to create. How do you go about implementing them? How do you document your styles and keep track that styles are being used correctly?
If you have personas, this is a good time to use them. What you need to create are some user scenarios for each document type and personas can be very handy for doing this. Describe the user experience for each document type. What will a user want? Why will they read the document? How will they use the document (reference or read front-to-back)?
And now you need to describe the style (or tone) of the document. You need to describe the style by setting down its definition. Something like this:
We use an informal style for this user guide; conversational tone, contractions (don’t instead of do not), second person singular, and we use words that would be used in everyday conversation.
Create a document now where you list each document type and add the user scenarios and style definitions for all the document types. This document should be kept separate from any content guides you may have for your documents, but it could be a chapter in your style guide.
Of course, defining a style and getting it applied is easier said than done. Applying a style is much harder if you are not the sole writer and rely on multiple SMEs. But, once you have your styles defined, it is easier to get others to adapt to the style requirements.