This exercise practices the first steps in the writing process, "Planning" and "Content":
Pick an open source project (or a subset of a project) that you think is interesting. It must be an area that you've never contributed to before.
Pick a feature that sounds tantalizing but is not clearly documented.
This might be part of a user interface, or it might be a programmatic interface (API).
Proceed to figure out how to use the feature by documenting its implementation and proper usage. Focus on getting a task done with one or a few features or interfaces.
Use the five step waterfall model to plan and define the content