Greetings!
First of all, I think on the premise: how to summarize the story I imagined in one or few sentences, for example; then, it comes worldbuilding, the characters and the decisions: who is/are the main character(s)? Their supports? Where do they live? How does this world/scenario looks like? (the latter in case of historical reenactments). Then, a title (provisionary, at least, for saving purposes). And then, finally, development.
For me, the process of character and situation developments comes freely; each character, main or support, has a fairly complex personality, which gives me quite some flexibility to whether one is going to be a hero, anti hero, neutral party or villain, and I let it flow as I see fit.
And I break it up in chapters, roughly with 5 to 20 pages depending on the scene that I am building up and where it leads. I hope I have helped you somehow ^^