For me, I start forming the main protagonist in my mind as I begin writing whatever idea popped into my head. It's slightly different for fanfiction, but the process is still similar. I then start wringing out side characters, other protagonists(some side characters count as fellow protagonists depending on where the story goes), and antagonists. For the original stories I tend not to post(yet), it's kind of one huge ass-pull. A lot of it, I need to refine and edit to make it all flow well.