Hemingway did it, and so should you. I’m not talking about drinking to excess or going on safari. I’m talking about writing.
Of course, our writing can take many forms. Like writing short stories, army lists, novels, and of course, what I call the theme side bar. Think of it as a scene, a vignette that reveals the flavor of your army. Better yet, think of it as a written trailer for your army. You, of course, have read dozens, if not more of these shorter than short stories. Some of them are highly memorable, others less so. They're all in those text boxes in the sides of your codices and rule books.
The one that I love is simple, and it sets the image of the 40k universe so well. Turn to your chaos codex, current edition.
There is the story of the desperate struggle of an imperial guard regiment under attack by the Red Corsairs. The Guardsmen cower in a bunker, and attempt to call in artillery on their coordinates, when a Land Raider rolls over their position, making a mockery of their resistance. The guardsman's death is an inexorable as a drowning man's. The Red Corsairs, finding him damaged and of no use, toss him aside contemptuously.
What elements does this need to make it work?
1. Brevity - the shorter you can make it the better.
2. Clearly defined conflict - the classics work best here, Man Vs. Man, Man vs. Nature, Man vs. Self
3. A satisfying resolution? - Obviously, you want your side to win. You're writing it, so you get to define the ending. Of course, this being a just a scene, you can build in a cliffhanger. Describe only a tiny slice of the victory.
Your hive fleet's lictor slaughters a few guards, but moves on into the night. Does it complete its mission? That's for the game and dice to decide.
You might a Dark Eldar Wych leaping into the fray against a tactical squad, performing a deadly dance with the marine sergeant. Have that fight end with the wych planting a poison kiss, and having it end with her asking who is next.