There was a time I was crazy enough to design a mobile app to get a job, which is probably something I would not do again. It is at inVision as a simulated prototype and the app screen flow is included here so you can see some of how I prepared for this app.
I began with paper sketches, did the screen flow, did some screens, did a screen-by-screen flow and during that discovered I had some online ordering issues that I wanted to do in a better way, so I did not finish that for this exercise but in reality finding out prior to coding it would have saved a lot of money and aggravation, a great example of why you should have a UX designer on your software team.
Be sure and check out the simulated prototype at inVision. This is something we can do for startups seeking funding, prior to any code or development dollars being spent, which can be a significant cost savings. This type of prototype can also be used to conduct user interviews or even user testing to get input for design changes before incurring development cost.