When working on product design:
First you want to get the "architecture" right (E.g. what are the core areas/screens/patterns)
Second, the UI metaphors (e.g. Are lists, feeds, carousels etc the right thing to do in each situation)
And finally, the fine grained details (is it 'your stuff' or 'my stuff', is it 'overview' or 'summary', is it 'done' or 'complete' etc).