Using English, write down some stories. Suggest some scenarios to fully illustrate the stories. Split the scenarios into sequences of contexts, events and outcomes. Reuse them to describe and illustrate the different behaviours of your system. Run the stories. If you need some code to make the system behave as it should, write down the behaviour of the code. Run the behaviours. If the code isn't doing what it should, change it so that it behaves. It's like testing, but without the tests.