Ooh, good question!
I usually start by writing down a vague outline of my characters and the plot. Then, I work on their traits and characteristics with mood boards and character cards, like this:
These are for my upcoming story, which I plan to publish in two weeks:
Mood boards and 'meet the character' cards
After I have a good idea of who the main characters are, I start thinking about their families, friends, and those who they will interact with often in the story. Sometimes, I make a rough list of the events I want to happen in a specific episode, but often, a random scene idea pops into my head while I’m working or doing something else, and I just write down that idea before I forget it, and save it for coding. If a scene I want to write requires a very specific background/overlay, I go on an intense BG/overlay hunt before writing the scene.