Try looking at the other Walkthroughs on the frontsite, as they should give you some inspiration. They aren't the greatest things ever, but they get the job done. Really, you just need some rough boss stats, important items, and the fastest way to get them. Keep descriptions and explanations short and as simple as possible, unless required otherwise.

If you go deeper, then you can rough out the patches in game plots (not that FFIX's is really difficult), or explain some of the optional challenges and recommended items. If you get something wrong, chances are, people will catch it, so there's less pressure, compared to the ones you buy for 30 dollars at the Gamestop.

If you use other stuff as references, try to make a bioliography, and give credit where due. It doesn't matter to the fan if it's at the end, but it does show a rudimentary appreciation for the work of authors.

Since you are writing for FFIX, don't use the BradyGames Guide. Most inaccurate bile I've ever seen.