In the next step, I print out a few of the photos and start making lists. I try to keep it to three categories: pieces of the costume I have, pieces I need for the costume to be effective, and pieces I want. The wants are extras that would be nice but aren't absolutely necessary pieces.
At this point, if I the "need" category is crazy long or full of weird items, I think about whether I really want to do the costume. With some costumes, I might decide that what I need is too specialized and I'll considering getting it custom made (like my Number 6 costume). That depends on how much I want it and how much cash I have in my bank account. ;) If I decide the list is reasonable, and I want to make the costume, I print this sheet out. By the end, most of the white space is filled with scribbles.
I start with the "need" list. I spend a lot of time (okay, more time) on the internet seeing what others have used and where they got their supplies. For example, the Kaylee costume above has been done lots of times. I already have a long list of links where I can get coveralls, patches, and a parasol from. I plan to wear this costume to Comic-Con one day this year. In fact, I'm hoping for a different costume every day: Kaylee, Princess Leia, Star Trek officer (new movie version), and maybe Poison Ivy.
And because I'm a nerd that likes to organize, I keep electronic files on each costume as well as paper ones in sheet protectors.
Do you have any special methods when putting together your costume?