A. Know your audience. Don't think that you're writing for experts and then give newbie advice. Or vice-versa.

B. Revise. Once you finish your first draft, you're about 10% done. Keep improving it until it's actually worth reading (not just by your friends).

C. Then announce it. The forums are littered with attempts at guides, most of which haven't gone anywhere, because their authors wrote part of a draft, got bored, didn't finish, and didn't revise.