As your confidence in the API increases, then you flesh it out. But the fundamental rule is, write the code that uses the API before you write the code that implements it. Because otherwise you may be wasting your time writing implementation code that won't get used. In fact, write the code that uses the API before you even flesh out the spec, because otherwise you may be wasting your time writing detailed specs for something that's fundamentally broken. That's how I go about designing stuff.
