>>150994938 (OP)
Actually write.
Don't fall for the 'eternal planning' pitfal. It's cool to have an outline and an idea of what you want to do, and even some key moments set in stone you want to have, but at the end of the day, the outline is not the story. So don't be afraid to come up with stuff as the story develops.
Golden Age Berserk was written mostly as it was being published and we all know the general opinion of that arc.
You might also want to write 'lesser' works first to get a feel of what you want to do. Try writing short stories or oneshots befores tackling whatever masterpiece you want to make.
And obviously, interact with the stories you like. Analyze them, disect them, find out what makes them great for you. A teacher once told me that a great writer is usually a great reader too, so yeah.
>Tldr
Actually write.