Here's a guide me and Civ posted a while ago...
Spelling and Grammar
One problem that many fan fics have is spelling and grammar usage. An easy way to fix this without memorizing the dictionary is to type everything up in Microsoft Word before you post it. The spellchecker will check for spelling and grammar mistakes. Also, make sure you read through each chapter your write before you post it, just in case the spell check missed anything.
Plot
Make your plot actually have a storyline, don't make it all about some random marine killing going around and slaughtering Covenant for a whole 20 chapters. It gets old really quick, and no one will want to read it. Also, one common problem with fan fic plots is that too many of them use the same storyline. Try to keep your storyline unique and unpredictable. Also, don’t do stories about Master Chief or any common characters from the Halo series unless you are sure you can do it effectively (so basically, don’t do it). And stick with the plot, no need to write a minor characters life story.
Cannon
We cannot stress this enough, please make sure your fan fic is within cannon. Do not make a fan fic about humans training hamster warriors to fight the Covenant or anything like that. A good idea is to check your work with sources like halopedia or the books and games themselves. If you ever are in doubt about weather something will fit, make sure you get it straightened out before you post it. WARNING! Halopeida can be edited by anyone, so use your common sense when you use information. Don't trust something stupid like "brutes are related to kittens."
Descriptions
Treat everybody as if they are a halo novice. Don’t leave anything up to the imagination. The more descriptions you have, the better, just stay on topic and don’t go overboard. Nobody likes to read “with their superior firepower, the marines overwhelmed the Covenant and won the battle.” Once again, reread each chapter and make sure you don’t have any bland areas. Don't add unnecessary descriptions.
Characters
This was covered briefly above. But you want your characters to be believable, which means: they need emotions, I would be your characters aren't robots, and that they need to be hurt, they aren't invincible, no one is. Also, we have heard a million stories about the ODST, mix it up a bit.
Common Characters in a Fic:
1) Master Chief
2) Some ODST
3) Snipers
4) Elite or Grunt (still not very used, so they would be okay, but don't make a million stories of them)
Setting
Every story needs one of these, it isn't a story without one. Where are the characters? What time is it? What does the sky look like? Think about those types of questions when you write, then add them in. It greatly adds to the full experience to know when, where, and also the what the mood of the setting is.
Paragraphs and Font
Make sure you indent each paragraph you write. Because the tab button wont work, three to five spaces will work fine. Double spacing is optional, but it is simpler to press [shift+enter] instead of taking up so much room. Some fan fics choose to use a different font as well, please choose one that is easy to read and doesn't affect your story. Every time somebody new speaks, you need to start a new paragraph.
Speaking
All speaking parts should be in "quotations" and all signing needs to be italicized in <brackets>. If the speaking part is over, you need a period, question mark, or exclamation mark at the end of it; however if you are simply pausing to say 'so-and-so said,' than you need a comma within the quotation. Ex: "I'm going to the story," Bob said, "and I will buy some cookies."
Punctuation
Don't make incredibly long sentences please. Instead, use commas, semi-colons, and parenthesis appropriately. It is not a good idea to list things unless necessary, as it makes the story much more dull and unpleasant to read. Please use apostrophes. When swearing is necessary, use three asterisks (*) and don't add any letters to the end of the word unless you are changing the context of the word (***ing grunt instead of *** grunt).
Tenses
One thing that is very easy to do is mix up your tenses. It is easiest to stick with a single tense throughout the whole story (usually past tense or present tense) but you may go from past to present tense, just make sure that this is clear to the reader. Do not make a paragraph like this:
One day Jimmy went to the store. At the store he buys candy, ice cream, and kool aid. Then he will go home and make cookies.
One day Jimmy went to the store. At the store he buys candy, ice cream, and kool aid. Then he will go home and make cookies.
One day Jimmy went to the store. At the store he buys candy, ice cream, and kool aid. Then he will go home and make cookies.
One day Jimmy went to the store. At the store he buys candy, ice cream, and kool aid. Then he will go home and make cookies.
ChucklesAnyone else here getting sick of tiny two-word posts that become huge because of the O.G. picture?