I start with the concept. What is the main idea.
Then I start thinking about the plot. Where does it start, where does it end, what happens inbetween?
After I got the plot down I start dividing it into chapters, I write down what I want to happen in the entire chapter, and devide those into smaller parts. In the beginning I only wrote a few keywords. But now that I'm further I often write the entire scripts instead of just the summary. That makes it a bit easier to know whether it would or would not fit in 7 pages.
And then I start sketching/ thumb-nailing. I often change a lot from what I've already written down, often rewrite what I already have too.
Sometimes I have random ideas of entire scenes too, I just write them down and try to fit them in the story. I have only 2 chapters written down so far, chapter 3 I have the main line and from there it is a blur till the end. But I don't know how long I want my story to be yet...