Customer first

First, understand the problem your customer has
And then you solve that problem;

Second, you understand the data. What data goes from where to where, how does it look, how is it stored;

Third, you create the API; how does the information look that is exchanged between customer and UI, and how is it between UI and backend;

Fourth
You don't need every UI detail, but you need to know which data needs to be shown how fast
Your whole solution changes depending on whether how they want to display on the interface.

Really
what does it mean to put the customer first?
put yourself in their shoes!
what's the data the user sees?
What does he input?
Where does the data need to go?
How fresh does it need to be?