How do I design my app

The most simplistic design you'll ever see has two buttons. That is the lesson. Take the notebook or the paper ledger that a shopkeeper used to keep track of who owes how much, and convert it into the simplest possible form. Nothing else.

As a designer, it is very difficult to think in such a simplistic manner. You will want to add ten features, because you can clearly see ten features you could add. But the act of stripping everything off, going back to first principles, sitting with the actual person who will use the thing and asking: what does he require on a daily basis? That discipline is where the real design begins.

The features can come later, layered carefully on top. But only when you have found the one true core of the thing.