I usually visualize the main plot points in my head, as if they were movie scenes (my current story has both a comic and novel version, so the visuals are important). Then I plan out the rest of the story to connect these key scenes.
When I was planning out my current story, I wrote out the background information for the fantasy world setting and the characters first, and then described the plot in a combination of paragraphs, scripts, and outlines. Since there are many flashbacks, I made a timeline of when events occurred and how old the characters were at the time so that I could use that as a reference when writing out the actual story. Then I spent a couple months writing out the entire story in script form (it was intended to be a comic first, so I wrote out what happened in every panel).
For the novelization, I referred to the original script (and comic episode if it was already published) and then wrote it out in prose form, sometimes modifying it if I wanted to add extra details.
