Creating the code from the design can be a fairly straightforward process if your design is reasonably complete. The sequence diagrams dictate the class interface and the state and activity diagrams guide the behavior of the member methods.
In a larger project you typically spend more time fleshing out the design before beginning implementation. In a smaller project, such as Phish you may find that you are eager to get to the code and that you make lighter demands on the design; keeping more in your head and less on paper.