The first thing is to realize there are two directions of game development you’ll need to plan out.
First there is depth. This is how development of your story elements are going to be planned out.
Second is breadth or width. This is how many individual branches you are going to allow each of your story elements.
I’ll provide you with an example and then comment after in general.
The example I am going to provide is a simple battle. Before you can design any mechanical structure you are going to need to determine the story elements of the battle you want to write about. These elements are the “rails” of your game and they define where the edges of the roadway are, where the road begins, ends and how many stops are in between.
This means that for each individual branching of your story, you’ll need a beginning, an end and however many “stops” you want to focus on during the battle.
Branching is customization of the story. I allow: a knife, a semi-automatic and a chain to be used for my first fight scene. For each story element involved in my battle scene, I need to be able to branch out each of the weapons. There are many other ways to customize a CS game and some of the better authors in our community have included quite a few ingenious ways of doing so. Zombie Exodus, Tin Star, Sabres of Infinity and Lucid’s many stories are but a few examples.
Keeping it manageable means understanding your limits as a writer first and a game designer second. As a writer, keeping battle scenes limited to a beginning, several stops/vignettes in that scene that will address plot points and arcs and the ending is all I can manage. At least right now.
I’ve found that most authors trip up with breadth - the more options you want to give the gamer, the more mechanics you’ll need to support those options. My suggestion is that you most likely will hear given to any beginner developer - start with a limited scope. Think of it this way: for every element of breadth you wish to add, you’ll need to design one mechanic and write 3x the words. Why three times the words? Replayability.