What's the best way to build a new airplane?

If you’re asking this question, you probably shouldn’t be building one yourself.
Are you asking out of curiosity, or is there something specific you’re thinking about?

Building an airplane is not like building a car or a DIY project. The “best way” depends on what kind of aircraft you mean (model/RC, ultralight, experimental kit plane, or certified aircraft), what country you’re in, your budget, and whether you want to fly it or just design it.