Easy. Write a script like you would for a movie, but indicate it page by page.
Look into script writing online, or even watch a tutorial on comic writing. It's all done with a script and a storyboard like films and plays are.
Writing page by page can work for really experienced creators but almost nobody in the industry ever does this. It's really easy to ruin it because you don't know how it's going to end. It's like the reader is writing it for you, because you don't know any more than they do.
Take the time to work out a plot. Get your "3 acts" or "4 stages" together, so you know how the story will end and what will take it there.
Create a script.
Comics are not the kind of medium you can just wing because they're sequential. A single art piece, yes, you can wing without planning. A comic? No.
Sequential art (film, animation, comics, novels or any form of storytelling) has to be planned for.