This is sort of a broad question. How would you answer someone if they posted something like "I want to make a costume but don't know how, help!" You, the cosplayer need to make some decisions first.
My suggestion is look at some EGL photos. People on the
EGl Livejournal Community are always posting, but you can probably find some at other websites. Find some pictures that you like, that aren't too complicated (to start with, unless you're already an advanced seamstress) and that you think will look good on you.
Once you've made some selections, post some pictures (or sketches) of the sort of style/look you're trying to achieve. Then people can give you better advice on what sort of patterns to look for (most of the patterns in GLB are for things like hats/bags- not dresses) and how to alter things.