I start with noting the idea down, then I leave it to let it simmer until I come up with something more concrete.
Then I write a very simple plotline (we start at X place, then X happens)
From there, I look at what kind of character I can push the furthest out of their comfort zone with the plot. Once I have their basic personality down, I look at motivation. This is where I start writing prompts for development.
I continue to develop my plot and my characters (and possibly worldbuild, in the case of fantasy) until I feel confident in my idea. That’s when I start my first draft.
When that is done, I print the product and mark out everything that needs more work. This is where plotholes become apparent.
I keep printing and editing until I am satisfied with the finished product, or I’m so tired of a specific storyline that I decide it’s fine as it is.