I usually start off with the idea.
I find something specific I want to portray most of the time (like the battle of Kursk), and let the idea form from there. I usually Google the subject, read up on it, and then Google it again for pictures. If I dont find anything that inspires me that way, I check a website called corbis.com that is essentially a library of photographs of....well, everything you can imagine.
corbis is kind of a mixed bag though. If you search for say, "Waffen SS" you'll get 40 or so good pictures of Joachim Pieper and friends at work and at leisure, and about 60 or so pictures of skinhead goons marching thru Pulaski, Tennessee to celebrate the birthday of the Klan. It's essentially a media source for everything, and you have to pick thru it to find what you want.
Once I do that, I'll make a few rough sketches of what I want to do and try to gauge the size of what I'm going to work with.
Once I find the size, I try to find the right base for my dio. I usually go to HobbyLobby or Michaels, I'm not sure if they are international, but you probably know where your local craft store is anyway, right? I can usually find a base there for less than a dollar (U.S.), and then I stain it myself.
Once I do that, I take the base and copy it on a copier at work, and usually make about ten copies of it. Why? because that way I can get a feel for what I actually have room for, and what I have to take out. Oftentimes I find that what i start with becomes something different entirely by the time I actually get around to making the dio.
As far as what comes first, the dio or the model? I usually start with the model, work on it until I start to feel even the slightest bit irritated with a difficult bit, and then work on the dio some, switching back and forth. It keeps it fresh.
If you get bored or impatient, just stop and walk away from it. Go start another model, or do something else, because if you hurry thru it just for the sake of getting it done, you wont like the results. Go check this out;
http://www.ww2modelmaker.com/modelpages/SGstreetbat.htmThis guy is a friend of mine; actually, he's the guy who got me hooked on modeling again after 20 years....My wife just loves him to death!
Anyway, he works on so many different things at once, it's unreal. He has stuff that he told me he has walked away from for over a year, and then he comes back and finishes it in less than a day.