Taken from TauOnline
Writing fluff is often a hit-and-miss affair. It is difficult, some may say impossible, to specify exactly what makes a good fluff piece a great one, and what simply makes bad fluff. However, I will endeavor to provide some guidelines:
1) Know what you're writing about.
If all you know are Marines and Orks, write about Marines fighting Orks. If you only know about one army, or only about "allied" armies, then just make one Chaos.
For example: If you know about about Sisters of Battle and Guard, make the Guard rebels, or Chaos traitors.
2) Size matters.
Short sentences can be effective, but not always. Try to give good size to your sentences, and break up the story with paragraphs.
For preference, a story should be a good length. I try for at least two sides of A4 (size 12, Times New Roman).
Be varied in your wording. If you don't have a wide vocabulary, then try writing with a Thesaurus at hand!
However, it is always important to use the words correctly. For example, "decimate" does not mean the same as "destroy", though some people believe it to.
4) Know about Fluff and Rule discrepancies.
Fluffwise, a Marine should have straight "6" for his Stats. Rulewise, he does not.
Try to find the key "game errors" which should be written out in Fluff. Marines are the main subject of these errors, so I will list them below:
Marines are rare. Just five or ten are enough to turn the tide of battle. Entire armies can, will and have surrendered rather than face a dozen Astartes. On the flip-side, most Guardsmen know that Marines are not positioned on a battle line unless it is going to face serious assault...
Marines are also extremely hard to kill. They can survive crippling wounds, lose limbs, be impaled or suffer a variety of other injuries and keep on going. Just about the only way to kill a Marine is to blow his head off!
These guidelines apply to Chaos Marines as well, though with some slight differences (Chaos Marines think more about their own skin than their Imperial counterparts do).
5) Write balanced fights. It can be hard (especially when writing "proper" Marine fluff), but try not to make the story a landslide victory for your favourite force. Marines may be able to slaughter entire squads of Guard, but a Crisis Suit or Aspect Warrior squad should be able to put up a good fight...
6) To fight, or not to fight...
Some of the best 40K fluff has little or no fighting. Remember this.
Of course, this guide isn't perfect, but I hope it provides some small measure of aid. If other veteran fluff-writers can chip in their comments, we could have a pretty good guide here...
I probably don't write enough, but logic goes a long way... here's a few off the top of my head.
7. Pay attention to mechanics. By mechanics I mean spelling, grammar (spelled with an a, not an e), and all that other stuff you (hopefully) learned in school. Here's the thing: good mechanics does not necessarily make a good story, BUT bad mechanics will always make a bad story. There is very little that kills the immersion into a story faster than constant mistakes.
8. As a rule of thumb, unless you're describing a document of some sort in your story, don't use numerals ("3"). Use the word for the number ("three").
9. Some may disagree with me on this, but... don't use special characters (such as Macharius, Lelith Hesperax, etc.) in person or as relatives of characters in your stories. Mentioning them is okay, but the fact remains that they are not your characters. They are more or less public, and unfortunately for the quality of fiction, too many people decide to use them in stories. First off, your Eldrad Ulthran (or whatever) may be different than somebody else's, and second, it smacks of laziness. Having relatives of special characters is just as bad; to have a character be the offspring of someone famous is just too convenient to be plausible.
10. If you're not writing an entirely serious story, some of these rules can be ignored.
11. Don't write a story from the perspective of a god (including C'Tan), a Tyranid, or a Necron. It gets real boring real fast.
12. Be original. You'll have less competition that way, so if you're good then you'll be the only one in your class. If you're not good, at least you'll have novelty value going for you.
13. Don't go overboard with the Ork speech. I know it's fun to write and all, but the fact remains that the first purpose of language is communication. Just make it Orky enough to get the feeling; don't go so far as to make it impossible to understand. After all, if your human reader can't understand it, how is an Ork supposed to?
14. Plan ahead. When writing, you should always be aiming for something, whether it is an event, an effect, or a destination in the story. Good plot isn't something that you can come up with on the fly.
15. Avoid "Mary Sue". To put it simply, a Mary Sue is an idealized avatar of yourself. If you want further explanation, Google it; you'll find countless writings and litmus tests on the subject.
16. No battle reports. One of the best things about writing fiction is that you can write anything you want. Don't limit yourself by trying to follow a battle.