We design our documents for readers. It is important to understand who they are, and why they might be using our document.
You want as thorough a characterization as possible.
How old are they?
Are they students?
Are they employed?
If employed, what do they do?
What do they actually do day by day on the job?
What is their educational background?
You should spend some time with them if possible to really get to know them.
You might have them fill out a questionnaire
People often know something about the subject of your document.
You should determine what they know and to what depth they know it.
This will enable you to write the document to their level.
Research shows that people use information in at least 4 ways.
This is reading to perform a task.
An example is the set of instructions (documentation) for assembling something.
Textbooks are written for this
They often proceed from general ideas to specific details.
This is often just scanning to get an idea of the document's contents.
Pictures are particularly useful here.
This is reading to learn how to perform a task.
You might be reading this document to learn how to do an audience analysis.
Observing what questions readers/users ask often can help make a document better.
It is somewhat paradoxical that the more writers know about a subject, the harder it is to anticipate (and hence write to) users questions.