DIY chapters have always been popular, and with good reason. Whilst it may be fun to field frothing blood angels and ferocious Space Wolves, the rigid and merciless attitude of some fluff-lovers leaves you with much less room for customisation then a DIY chapter. You can create your own history, tactics, army composition and battle tactics. The only real restraint is the number of founding legions which your chapter has branched from, and the a few elements of official fluff which ties together the entire universe of Warhammer 40k.
1: Coming up With a Theme
The appeal of DIY chapters is that your creativity can run wild with your own fluff and colour schemes, tactics and practices. But what is it? Successor chapters always share at least a small amount of characteristics with their parent's and from this base you can build up a gripping and genuinely fascinating background. Here are the basic steps of Theming your Space Marines or Sisters.
A-Who have they been given their gene seed from?
Once implanted into human, a Primarch's geneseed will change them both phyically and mentally. They will adapt certain likes and dislikes, pros and cons, nasty and nice from the Primarch. This is what gives a chapter its outline, Leman Russ, for example questioned the Emperors might at first, and although the Space Wolves serve the emperor with arguably more vigour then any other being, they still remain rather begruded against the Imperium as a whole and retain many of their native, tribal customs. We can find complete opposite of the inhabitants of Fenris in the Ultramarines, who have followed the Codex Astartes to the letter at all times.
B-How have they chosen to differentiate themselves from their Parent Chapter?
When a new chapter is founded, it will have a new colour scheme, chapter master and equipment assigned to them. They can customize as they see necessary and are not in any way controlled by the legion which they derived from. As of such, they will obviously grow there own, unique tactics, appearance and flavour. How do you want them to act? When their officer says ''jump'' do they ask "Over which cliff?" or "You lookin' for a fight?” Are they rigid followers of the imperial propaganda and unforgiving followers of the machine god? Do they have their own minor gods? Are they a relatively new chapter or a rather old one? How do they see themselves? All these questions can make or break the theme of a chapter.
C-Additional Bits
When writing up your tome of knowledge dealing with our army, you will be coming up with ideas left, right and centre and jotting these down for later are well worth it. Their grudges, experiences, past battles and preferred soft drink add just that little bit extra to the uniqueness of its feeling.
2: Where to Now?
I imagine that most will have at least bought a box of tactical marines by the time they have gotten halfway through the work (or play depending on how much you like MYOSMC) I have described above. This is good as you can try out sample colour schemes and play a few mini games before committing yourself too much. If you don't like the theme and way your army is played so far, it is no big deal. Not too much money lost and no sleep lost writing a novel based on the adventures of their chapter master. If necessary, retrace your steps, if not, follow me....
3: Making your Army List
Theme? Check. Colour Scheme? Check. Time to draw up a few army lists. These should always stick with the ideals of your chapter. A Space Wolf successor is not going to suddenly go all shooty and a Dark Angels force isn't going to get into a hyper-active, kill and/or maim melee frenzy on the battlefield. Draw up army lists for 500 pts, 1000 pts and 2000 pts. Ask yourself what are the key differences between the different points values army composition, apart from more models!
Start off small, then make a copy of that small list and expand on it. Get all your different lists examined by our veterans and your fluff scrutinised by the lore masters of old. How have the responses been? Good/bad/ugly? If bad then take in their advice and apply it within you chapters theme limits. Once successful, your ready for the next step-BUYING SOME MODELS! Is that some sighs of 'Finally!' I hear ?...
4: Bob the Army Builder
Time to steal/beg/earn some money and buy some models. Stick to your favoured army list and wait for later to buy other models. Do not take on to much at a time and build and paint your models before buying new ones. This way you are managing the work you have in front of you, and not hurting your wallet to much-at least in one go. Once you have built a 500 point army, play some friendly skirmishes for practice and some competitive battles to flesh out any tactical and compositional weakness you have. Once you have a handle on a smaller force, build it up to the next level and repeat the preparation and practice steps described above. Once you have reached your desired 'force size' you can start all over again!
Thanks to JJ for sending me this 
