The invention of such classes and their combination with pre-existing classes to form a working system is an incremental process requiring many trips back and forth from high level design through implementation. As in many other methods, you start at a high level, produce a rough cut at a design, then immediately begin implementation. Selected subsystems are targeted, usually not the easiest ones. This provides a reality check on the design. Then, back to the high level to revise by what you have learned, return to implementation, and so on.
Implementation throws the cold clear light of day on design, design guides implementation.
From time to time you make a side trip into system use scenarios. These do not direct the organization of the system, but rather test the evolving design. Sometimes it doesn't, so you go back and figure out what additional useful ideas might be wrapped in classes. The use scenarios are accompanied by object scenarios, showing the interplay of objects to accomplish the use scenario. A novel graphical notation is used, which allows easy depiction of interactions between many more objects than the conventional ladder or lattice-like interaction diagrams often used elsewhere.