This "How I Built This Branched E-Learning Scenario" article outlined a similar process to what I used for this.
https://community.articulate.com/series/nicole-s-articles/articles/how-i-built-this-branched-e-learning-scenarioI simultaneously made a slide branching map in PowerPoint to gather my thoughts on how the branching should function and then also wrote an outline in a Google Doc of roughly what would happen in the story on each slide and what kind of options I would give people on each decision slide. Once I was happy with the outline and map, I took the outline document, wrote out my full script in it, and then transitioned over to Storyline for development.