It's time for you to come up with a project of your own, something that you can work on by yourself; we step through the development process:
Think of a project of approximately the same complexity as the one described in this chapter. It should be a single module or a few modules in a single package.
Imagine that the project is already done, and you need to write a description of what you've done, along with a little bit of demonstration code. Then go ahead and write your description and demo code in the form of a doctest file.
As you're writing the doctest file, look out for places where your original idea has to change a little bit to make the demo easier to write, or work better. When you find such cases, pay attention to them! At this stage, it's better to change the idea a little bit and save yourself effort all through the process.