No matter what type of game you’re working on, for a writer the iterative process needs to start with some kind of relatively simple story overview to ensure they are beginning the development path in the right way.
This might well stem from an initial meeting with the design team in which ideas are thrown about, in keeping with the main aims of the game’s style and gameplay mechanics. It may begin with a brief from the project lead based on initial ideas they already have. Whatever the trigger, this initial overview should be seen as the starting point of the whole process.
This first layer of detail is likely going to be just a small number of pages that define an overview of the main story ideas along with notes on the main characters. This should be presented with the gameplay in mind to ensure that everything sits within the scope of the project.
The first, very brief, synopsis should also be included in this high level overview, which should take into account any plans for branching storylines and/or multiple endings if appropriate.
It can be very tempting to let your creativity run away with you and deliver much more than you need at this stage, but it’s really best to hold back and put this into the stages that follow.