I basically learned by doing. Just took screencaps of the show and played around trying to make the characters look good. The more you do, the more you'll be able to know how certain things are supposed to look on the characters, and can make better vectors with less and less info. That's what I did anyway, I'm sure by now there are tons of various tutorials you could hit up for more information, but the best way to learn is just to try and do it!