You need to write a module. You write tests that tests the functionality of the module. But to pass those tests you need a couple of classes. You write tests for the classes (or at least the one you intend to work on next). The class needs some methods, you write tests for those. You write code for the methods until all methods tests pass. Hopefully the tests for the class pass, otherwise you might need to update methods and tests for those. Unt so weiter.