First Issue, well... Issue.

All the rules of telling a story still apply.

You might find it helps to look at the way screenplays are structurally constructed, since there's an obvious visual link (you are writing for the next person (or yourself) to interpret what you wrote - as opposed to just writing as a straight tap into the reader's imagination).

You can start with the origin, but you might find it easier to sell if you launch straight into action and then at a later stage deal with the origin. Of course, if you're gonna self-publish, you have more freedom to do what you want.

Break your story down into page/scenes, then flesh them out from there. I thumbnail the pages even though I am not gonna draw them. Then I write the script proper.

Good luck.