Ok First of all pick a subject you are knowledgeble about, Now there are 3 parts of any good guide
General introduction:Give the Reader a General understanding of the subject. E.g. If im going to write a guide for how to make a good Twink guide i might what to define The meaning of Twink for my reader.
Items Needed: Not only listing the items required but giving links to a wow database like thottbot, wowhead, etc and going into detail about how you get them.
Bulk of the item: Make sure you do not miss-spell anything that might be important (E.G. +15 agility Enchant) And be sure to make every point understandable to your reader.
Thanks for reading my guide to guides, Enjoy! twocents