One of my professors who had been to Cambridge University for one study term back in the 1960s told me that in order to receive a degree in certain fields from the university, there were few formal requirements. Other than taking a certain exam, if one were simply hanging out in town for a certain period, it sufficed as proof of academic enrichment.
Now, one way to go about learning the craft of writing is to hone specific skills that produce good prose. After all, writing is an act of production.
But a compelling case could be made for quite a different approach: immerse yourself in the best that has been written.
Study the works of the finest prose writers (past and present), essayists in particular, but also the best long-form journalists and writers in various genres—science, history, philosophy, world affairs, culture and so forth—and even some practitioners of fiction.
Study their word choice.
Study the cadence of their sentences.
Study their reasoning.
Study how they suture sentences and paragraphs.
Study how they start and how they end their work.
Study how they create mood.
Study their voice: how different writers strike different notes.
The wider your exposure, the more resources you can summon for expression, the more adept you will get at using them. For instance, there's a word in English: 'prudent'. The dictionary defines it as 'acting with or showing care and thought for the future'. However, it is only after you have seen it used in a dozen contexts that you can yourself learn to use it confidently and discerningly.
It is one thing to know five common techniques writers use to arrest the attention of their readers at the outset. It is quite another to sub-consciously absorb the deft exploitation of these tricks made by the masters of prose.
Seeing is not just believing. In many places, seeing is becoming.