I don't want to make a year-long "dungeon" or at least not one over a year. I don't run individual "dungeons" of any great size in any game; I get bored with them as GM very quickly and just want them over, and they tend to have a point or goal attached to them which I want players to get to reasonably quickly. I also want to produce something that's actually going to be useful to me - there needs to be a response every day to "why should I even bother with this".
Therefore I won't be building a massive dungeon over a year - the plan is to build something with a consistent background, which I can take bits from and use independently if necessary, but which has some link between all the parts. I'll also be using a sci-fi planet-hopping setting.
I need to have some themes/prompts set up or a way of generating them, because otherwise I won't know what I'm going to do. At the very least I should have an idea of how I'm going to start on the morning of 1st January when I'm hungover to all hell. My experience of challenges like NaNoWriMo is that absolutely the number one thing you need to make sure you have is to know what you're going to write the next day.
A list of prompts helps with that at least. My current plan is to work through the twelve biomes defined for Starfinder, one per month, and work out some way of tying them together as I go. Sci-fi games have high PC mobility after all and don't need to have regions physically connected when you could just zip to another planet - you do need a reason to do that though.
Per week, I might draw a card from a standard 52-card deck, or I might use one of the location generating systems I have, or I might already have an idea based on what I did last week. I'm not going to formalise that. Each week, though, I want to have a location that needs developing based on that month's theme which is tied to other locations probably, so the first day of each week will mostly be spent devising that location, and then I can fill parts of it in as the rest of the week goes on.
I do need a notebook for it. I have a lot of notebooks but right now I plan to use the Squarehex gamer's notebook that I have, which has 50 pages that give spreads with lined and squared pages. That means I'll have to do more than 7 days on a spread sometimes, if a spread should be around a week, but that's perfectly possible - two A5 pages is a lot of space if you are concise.
Each day's goal will be to define a "thing" in the world - something specific that can be encountered and played through in itself, a room, a complex social obstacle, whatever. There isn't going to be an expected word count, but the format will in itself restrict how much I can write for each room, which is deliberate. I should only need to do a minimum of a few lines of writing every day; otherwise I'll hit a bad patch and give it up, and I'd rather not do that.