There is a lot to consider when starting to build an application, and you might be surprised to learn that programming the application is one of the last things on that list. Before I sit down to start building my iOS application in Swift, I'll first have to build a plan. In your own work, I strongly suggest following this approach as well. It will ultimately save you time in the long run, and hopefully prevent you from spending days or weeks on a bunch of code that you'll have to either throw away or heavily modify when an unplanned change is made.
Please note that this is an imprint of an article that first appeared on Learning Swift (http://swift.staticred.com).