Dear subredditors, how do you cope with software project estimation? What’s in the first place for you — the development process or its evaluation, feasibility testing, and other stuff?

I'm just wondering how you guys approach the workflow initially before the first line of code is even written. I've read through many software project estimation templates, but still can't quite understand where to begin. Here the project itself and programming language don't matter. What I'm concerned with is the priority of tasks. Where should I begin developing a smartphone application? (Swift, if it matters).