Once you have chosen an army that you would like to play, you need to develop the character or "flavor" of your new army. You may decide to base your army on an existing army (Ultramarines, Blood Angels, Alaitoc Eldar, Catachan Jungle Fighters, etc.) or you may want to develop your own unique army. The later method is often called a "Do It Yourself" army (DIY for short). This article offers some ideas and tips on developing a characterful army.
Tip #1 - The Hook
Every DIY army starts with a hook -- something that distinguishes your army from every other army on the battlefield. Perhaps your army is on a crusade for some past grievance, or maybe your army is led by all veteran officers. Your troops may paint a part of their uniform a certain color as a reminder of some great victory, or maybe they prefer to fight in close combat and always charge into battle. A hook can be anything.
Tip #2 - Structure / Organization
Your army may have some unique organization that varies from the "typical" force. Squads may contain a specific number of troops or certain units may be unavailable to your army (no heavy support choices, for example). The key is to develop some rationale for your organization (your chaos marine squads contain 8 members because that is the sacred number of the chaos god that your army worships).
Tip #3 - Color Scheme
Perhaps the easiest way to distinguish your army from all the rest is to develop a unique color scheme. Be creative with your colors, but keep it simple (you will be painting LOTS of figures). If you want something really unique, go for it. Everyone knows that Orks are green, but that does not mean that yours have to be. Maybe they wear red face paint (a rip off of the blue paint worn by deathskulls), or maybe their skin is dark brown (because they live on a peculiar world with a strange ecosystem). Even standard colorations can be varied paint your Blood Angels Red Gore instead of Blood Red. Anything to make your army stand out.
Tip #4 - Characters
Every codex allows you to create "special" characters that ARE tournament legal. How you may be wondering?? It's quite simple. Choose a standard character and then give them equipment from the armory. Maybe your commander wears artificer armor and always carries a master crafted storm bolter. Alternatively, your character may carry a trophy from some great battle that they fought (a warboss head, a genestealer claw, a space marine helmet, etc.). You can also give your character a name and a history. Write-up a brief biography -- famous battles, awards, defeats.
The important thing to remember with your "special" characters is --- Don't get attached to them on the battlefield!! You may have spent hours creating a beautiful model and associated history only to worry about your character dying in his first battle. If your character does meet an unfortunate end, it doesn't mean that you have to start from scratch and retire your old model. It is highly probably that your leader may be seriously wounded (hence, ending their usefulness in the current battle), but with a little rest and medical attention they can return to fight again (and with a vengenance!!)
Tip #5 - Story
Develop a story for your army. When were they formed? How? Under what circumstances? Who are there friends or enemies? What is the battle history of your army? Where are they from? Describe their homeworld? How do they recruit new troops? How do they travel to different battlefields?
Your history does not have to be extensive, it can be a simple paragraph or two. I would encourage you to write a history for your army whether you have a DIY army or an existing one.
A Note On Special Rules
This last tip is more of an opinion, and a very strong one at that. There are many, many individuals that enjoy creating all sorts of "special" rules that apply to their army. These individuals see their "special" rules as characterful and the ultimate expression of their army's uniqueness.
I would strongly caution anyone against using any homegrown "special" rules (except in a situation where you are playing in an established gaming group and have ALL agreed to playing with homegrown rules). You can develop a flavorful army working within the rules in the 40k rulebook and your codex. The trick is to limit and/or maximize certain aspects of your army. If you are a fast attack army -- take three fast attack choices and no heavy support. If you are assault-oriented, take lots of close combat specialists. Etc, etc.
The problem with "special" rules (even the ones that are created with the best intentions) are that they tend to de-stabilize the game. Games Workshop has spent countless hours developing a game system that is balanced and well though out (this statement will lead to endless arguments as to whether the rules ARE balanced, but I honestly believe the intent is there). A "special" rule that you have created for your army undermines the core game. Plus, it is very difficult to find someone that will allow your special rule in a battle.
It is important to distinguish "special" rules that apply to your army and "house rules" that apply to your game group. House rules are certainly ok, so long as everyone knows the rules before hand (an excellent example of house rules can be found in the back of the 40k rulebook -- "The Ultimate Secrets . . ." section is full of rules interpretations and additions)
