There's no mystery to it. It simply involves the orderly transfer of thought to paper. Writing well is hard. It requires undivided attention, clear thinking, organizational skills, and lots of practice. But there are enormous compensations for all the effort required.
Good writing is the most direct, effective, and eloquent way to communicate.
It allows your meaning to exist in durable, accessible form across space and time. Furthermore, it's essential to the development of complex thought. Most people tend to think of writing as nothing more than putting speech or thought down on paper. And these mental faculties actually enhance your ability to analyze and synthesize the matter at hand. Some good scholars of my acquaintance claim that they don't even know what they're going to write about a particular subject until they've literally begun to do it.