A specific version of this is just writing a single connected "skeleton" of app, stubbing out absolutely everything that isn't needed with 'if (input == this) r

A specific version of this is just writing a single connected "skeleton" of app, stubbing out absolutely everything that isn't needed with 'if (input == this) return that; assert(false);' and just getting something from input to output. Then it's a lot easier to go and add more meat.