I usually start by drawing the character concepts as I think about the story.

then I talk about my ideas with people I trust (and who want to pay attention, this is the hardest part haha)

then I make a mental map of the story, like a graph with concepts, arrows and things that will lead to each event.

then I start drawing the script on paper, like a storyboard.

it's easier for me to just draw the comic script, instead of writing. always using pencil and eraser, because I tend to erase a lot, and redraw panels, layouts etc.

when I'm stuck I try to write again to play around the ideas.