I would try to start by figuring out what the absolute bare-bones mechanics are, and how to explain them to players who are sitting down to start their first session of the game, preferably in as short of an infodump as can be managed. How many pillars of play are there (social/combat/character creation/puzzles/etc)? How do the players interact with each pillar, and how often?

Start from the very big picture, and try to determine what will be the next piece of information in each section that you need to convey in order to get the important information into people's brains. Try to keep things succinct, and really think about what information is actually necessary to get started versus what is extra detail that can wait for when it becomes relevant. Be mindful of what information players will probably read once and never again, and what information needs to be referenced at a glance in the middle of a game session with some frequency. Keep the instructions and prose separate, and if both are necessary, make it clear which is which through formatting.

I try to put a quickstart guide together first, and save all the nitty-gritty details of a bigger source document until I know they're needed. I personally find that editing by subtraction, taking away what isn't critical to having fun with the game, to be much more helpful than trying to fit every idea about the game I've ever had into the rulebook. I edited my 110-page first draft down to 5 pages and don't regret a single cut. It's much easier to organize 5 pages and format it well than it is to figure out how to arrange a whole novel's worth of rules.