First, make sure you have read Chapter 6, “Designing Illustrations,” in The Essentials of Technical Communication and then consider the following questions.
What kinds of documents will you create as a professional in your field? Which of these kinds of documents will need to include illustrations?
What kinds of illustrations will you need for these documents?
In what contexts have you noticed the most helpful illustrations? The most distracting?
What kinds of documents will you create as a professional in your field and what kinds of illustrations will you need for these documents?