For me, an idea usually starts with me blasting music on my phone and coming up with cool story scenarios from the sound of the music. If I like the idea a ton, i'll take a mental note of it. I like to start drawing the characters, which can really give me a lot of ideas about them and their possible role in the story and gets me super excited!- I usually have a very vague idea of the story and characters at first.
I then usually start a google doc, where I will start dot pointing ideas and building off them. I always keep in mind that it will change, but I keep going and writing ideas. I will often rewrite the beginning outline later since I figure out what needs to be established. I continue this until I have a very solid outline of the plot with clear reasons for each scene and pay offs.
I then move onto my script, where I follow my outline (having the completed outline means I don't stress about what happens next or what the characters say.) The script will always have a first draft where I'm figuring out what characters say and what happens, and I will go back and edit to make sure the dialouge fits the characters, is relevant and is interesting. I continue this until the entire script is done.
Once the script is done, I go through and edit the entire thing because I get to see how scenes lead into each other and whether or not something works. I usually re-write the beginning because I have a clear view of the story, and I will add/remove scenes if needed.
With novels, it's very similar except writing takes me a lot longer and there's much more heavy editing needed because with scripts no-one but me needs to see it