Obviously I cosplay as characters I like, but sometimes I'm captivated by the costume/their outfit first (like, "HOLYSHITWHATTHEFUCKISGOINGONIWANNACOSPLAYTHIS"), then I delve into the series or game to learn more about the character. I also love a good challenge and enjoy the learning process as well, so I usually choose something that looks difficult (for me) and try to break it down with an initial sketch.
Depending on how many reference images I have (either ones I scan from artbooks, screencaptures I take, images taken from google, etc...), my flat sketch can take 5 mins or 3 hours. This is the most important since I figure everything out--list all my materials, draw in all the pattern pieces I'll need to cut out or create, write what I need to assemble first--it's the blueprint. I'm always reading tutorials online or going to fabric and art stores to browse new techniques or different ways to use the same materials I've been using. Sometimes I don't always know every single fabric I want to use for the costume, but I do have a general idea of its thickness (depending on the character sketch), and how I'll be cutting the outfit.
So let's use my Zephie costume for this process outline. The sketch took me about 2 hours to complete but I spent probably 6-8 hours compiling images and playing the game to understand this character and find the appropriate pictures I needed.
(closer view. It helps a LOT to have experience in garment construction... and also go through a lot trial/error, heh)
This is the most important part, the brainstorming/blueprint. I'll write more about pattern drafting/draping later this week.