This is my first Instructable, so please feel free to ask questions or comment or whatever. I'm a very OCD costumer but I'm also certain that the same methods I used could easily be applied to whatever you wanted to do. Because of the complexity of the costume, most of the individual steps could have been Instructables in themselves; but I've tried to be concise and clear with the explanations so you can apply my methods on your own projects.
I've been costuming for about five years now. For some insane reason, I had been jonesing for something complex and challenging enough to make my brain melt out of my ears. So, I actually sat down and tried to THINK of something really hard I could make. Finally, going through a folder of images, I found a design I'd previously dismissed immediately after looking at it as a real pain because it had at least two things I didn't know how to make - armor, and giant horns.
The character in question is Mystic Lord Orlouge from the Square RPG SaGa Frontier. It's an older game, from back when there was still a pretty large gap between concept artwork and the in-game graphics. If you've been playing games for longer than a few years, you probably remember the mild disappointment when a game didn't exactly match with the explosively fanciful cover artwork. In the game, Orlouge is a moderately interesting little super-deformed villain. Tomomi Kobayashi's artwork, however, paints a much richer picture than the practical confines of the game.
The great thing about working from illustrations is what happens if you think about the restrictions on other types of designs.
A) Live-action, music, etc - has already existed in reality and therefore cannot break any laws of physics. (Any person who's done live-action costumes knows that finding the exact that someone ELSE used is a huge pain, though.)
B) Animation - can break laws of physics from here till Doomsday, but has to be simple enough to be re-drawn say 32 times a second.
C) Comics - can break laws of physics and vary from page to page, but still has to be drawn multiple times (on a deadline).
D) Illustrations (for novels, covers, artbooks, etc) - only has to be drawn once, can make Newton roll over in his grave, and doesn't have to make any sense to anyone but the (one) person drawing it.
So, obviously, if you're looking for a sanity-killer, illustrations are the way to go. Thanks, Kobayashi-san! ;)
Step 1: Breakdown
There are some unique challenges when you're working from an illustration. The great thing about the "vagueness" of art that doesn't have to be redrawn (depending on the artist's style, of course) is that it gives you room to make some of your own choices. I'm primarily a cosplayer and most of the time only do recreations of anime, games, movies, bands, etc; so creativity is a real luxury in a hobby comprised of imitation. The bad thing is you have to figure out what the heck that squiggle is supposed to be, and what that means about the squiggle next to it, and when to ignore it because it really IS just a squiggle and isn't supposed to be anything at all. This can be especially infuriating when you may only have one reference.
For this project, the images below comprise ALL my references. I had two illustrations by Kobayashi and the in-game sprite (represented here by a sexy photo of my TV), none of which quite agreed with each other on the details. Often, you don't have ANY reference for what a costume might look like from behind. Basically, you start with the elements common to your references, then analyze the ones which are similar (and pick what you prefer), and resign yourself to (or celebrate) making up the remainder.
Once I have a fairly good idea in my head, I usually draw my own references. They don't have to be coherent, and they certainly don't have to be very artistic; but they're very useful to refer to when you're trying to remember how YOU wanted something to be. I usually write my notes on what techniques and materials I want to use on these. I also print out the most useful references I have so I don't have to keep running to a grab a book or my computer to check a detail when I start working.
I'm not going to list this as a separate step, but it's really important to figure out which elements of your costume need to be purchased; supplies and things like shoes or wigs. Everything you need to order should be ordered early in your process. Even if deadlines don't really matter to you, it's profoundly frustrating to have to wait for something to arrive.