Abraham Lincoln said: "If I had six hours to chop down a tree, I'd spend the first four hours sharpening the axe".
The same applies for building an app.
Draw a schematic of data flow.
Start thinking about data structures for your app.
Write test cases for imaginary modules that talk to these data structures.
Code the modules utilizing the above test cases.
Write app code that utilized the modules.
All the simple programs have been written.