Writing a proper Guide
Seems somewhat odd to write a guide about writing a guide. But if you think about it, its not all that weird. Tutorials and guides make up a large part of many websites. Hence, most site owners give writing tutorials a shot. It is rarely considered that writing tutorials, just like graphics, need talent and basic skills. Of course, again like by graphics, everyone has a different style. I can usually recognize who wrote what tutorial on Neo-Reality just based on the writing style and not because they told me who wrote it. If you try, I think most people can recognize a difference in tone and style when Jen writes compared to when I write. Both styles work very well though, they are just different :).
Appearances
Your thinking "Not the lecture on writing in English -- again!". Yes, I know it gets old. So I will make it fast: Use proper English, real punctuation and spell at least 90% of the words right. Now, that wasn't that painful was it? Now I will get into more detail were the most mistakes are made. Some people seem to think its "cool" to write oddly. This doesn't necessarily mean wRiG4eing LyK 7hIs, which is a mixture of StIcKy CaPs and writing with numbers. I find it very annoying when people find the need to end every sentence with a explanation point (!). Explanation points symbolize that you are yelling or commanding. Now, we aren't at the army, so lets keep the commands tucked away in a corner. Another bad habit is to capitalize every word. This is especially annoying for people like me who speak English as a second language. In my mother tongue, German, every noun is capitalized. Hence, it is confusing for me to have you capitalizing everything since I am used either to the English way of only capitalizing proper nouns/the beginnings of sentences or the German style I just mentioned. It just makes it seem like you don't know proper grammar.
Now we come to the second half of appearances. You want your guide to look nice. Try to break it up into paragraphs or steps. If your information is just all over the place and completely at random, it is very hard to understand what you are trying to get across. Instead, your visitor will just get frustrated and leave. Also, try to make the tutorial pleasing to the eye. Keep things ordered, the font size easy to read and avoid extremely bright colors for the headers and so forth.
A Topic
You wonder why I devote a whole section to the simple matter of finding a topic to write about? Because this is where website owners, especially those of new sites, make their first mistakes. Part of the reason you are writing the guide will probably be wanting to increase the quality of your site and teaching your visitors something new. By creating another guide on a topic covered 40 on other websites already you won't be doing anyone a favor. Think of the 5 most common guides you have seen so far. The ones floating on the top of my head would be "n00b vs. newbie", a screenshot guide or "Basic HTML". While these guides might have been interesting the first 5 times I saw them, they have now gotten old. Of course, these tutorials are useful and all, but you should first go for the more creative tutorials. Why? Because there are many sites to read a n00b vs. newbie guide at -- but you might be the first one to offer a detailed guide on whatever. So consider what you know a lot about and try to find a topic that you have never seen a tutorial written about but that you think would be useful. Some topics haven't been written about because they are just very un-interesting or there is too little to write about -- like "A list of all pets with 4 legs". Sorry, but that's rather useless information your clogging your website with. If you need inspiration, check your comments and so forth if anyone has asked a question recently. You can try to base a tutorial off of the question.
Writing Style
Now, remember who your readers are. If you run a neopets fansite like Neo-Reality, most of your visitors will be ages 10-13. This means your vocabulary needs to be understandable for a 10 year old child that's not necessarily a genius. This doesn't mean you can't use words like "fantastic" or "enormous" since most people know them. Just keep off words like "fungible" or "sapid". Also, try to keep from sounding like your adding in big words just to sound smart -- trust me, it will just make you seem like a child trying too hard. Instead, use a natural tone and words like you would in every day life when talking to an adult (not your best friend when your pretending to be 4!). Making your visitor feel like you think (s)he is an idiot is also something you should avoid. It can be very annoying when everything, including how to press "file >> new file" is illustrated in a screenshot -- I can figure that one out without visual guidance most of the time, thanks xD.
Odds and Ends
Of course, like I mentioned before its good and important to have your own style. Just remember that all good things are in limits. A good tutorial should be rather long. Try to have at least 3000 characters (to show you how much that is -- this tutorial has over 5500!) in your writing. Use a spellcheck program to make your tutorial as easy to read as possible. Last but not least, offer your visitors a way to contact you if they have any questions, which you can do in this case at Kathrin433@yahoo.com