It depends on why you are writing a walkthrough. Are you trying to get experiences with writing walkthroughs that you can use in the future, do you just really like Pokemon Yellow and want to share that enthusiasm with other players, or is there some particular element of Pokemon Yellow that you think needs to be expounded upon? How specialized a guide do you want to write, and why are you interested in writing it? In general, the basic information that you would want to have in a walkthrough (plot, where to go, trainer rosters, encounter rates) has been covered in plenty of other walkthroughs by now, and you can find it elsewhere on the internet. Given that, I'm assuming that your goal in writing a walkthrough is to provide something that other walkthroughs don't provide. That could be clarity that you don't see elsewhere, it could be something about your writing style or sense of humor, or it could be some element of Pokemon Yellow that you want to explore or explain in more detail than you've seen done elsewhere.
I would start by outlining your guide and figuring out what your goals for it are. Don't worry about filling in the details right away; those will be easy enough to plug in later. If your guide is for beginners, maybe try to think back to what you wish you had known when you first played Yellow, or what you struggled with, and address those topics next. Then, start filling in the gaps, and try to remember the things that seem obvious to you now, but did not seem so obvious back when you started playing Pokemon games.
One thing that I like seeing in walkthroughs that I frequently don't see is team help, and perhaps information on what team the guide writer used (and what levels those team members were at for each badge). While Pokemon Yellow isn't a terribly difficult game, and can obviously be beaten with almost any Pokemon, it's nice for a beginner to have a sense of what he should be using (and warning if there is a sudden difficulty spike/level increase). I also like guides that give some information on newly catchable Pokemon as you encounter them, and give advice about how best to use them and how useful they may be in the future, both in-game and competitively. Having objective information is great, and trainer rosters and wild encounter rates are helpful, but that information is readily available these days. What are not so easy to come by are the subjective thoughts and experiences of the walkthrough's author.
That being said, don't neglect the accuracy of the details. I recall trying to train my brand-new Squirtle in Yellow on a trainer that my guide (a physical, published guidebook) said had a ground type like Diglett or Sandshrew. Needless to say, I wasn't thrilled when my brand-new Squirtle found itself face-to-face with a significantly higher level Pikachu, rather than the ground type I had been promised.