By this point you have your idea and you know what you want it to look like, and now it's time to start coding!
...not so fast
There's still a lot of thought that has to be put into the technical side of the project.
Before you start coding and bringing your idea into reality it's really helpful to plan out and design the data and classes you're going to use. Just like when you created rough UI prototypes, designing the data you're going to use will make coding a lot easier.
Knowing what interfaces and data are needed and how they interact with each other before you start programming will not only save you time but a lot of headaches as you would have already thought about many of the tough technical questions on how to create your project