Tested Application Step 2: One Feature, Test First

Take the smallest unbuilt feature on your list and build it entirely test-first. Write one test, watch it fail, make it pass with the simplest thing, then refactor. Repeat until the feature is done. Keep a note of every time the test forced you to change your idea of the interface, because that is the design pressure and it is the part people do not believe until they feel it. Do not write a single line of implementation the tests have not asked for.