Sethra007 asked From Start to Finish: How Do You Design A Campaign? so...
As Giles advises Buffy, we feel our way.
I generally have a length in mind. "Six weeks, like usual" or "an academic year and then I'm done, tops" or occasionally "this could run and run!... probably six weeks like usual but hey." I generally have a beginning, some things to go in the middle, and something resembling a likely end.
An elevator pitch, since this is for a games society where pitching at the start of the academic year is the traditional method of getting new players. Old players will also be consulted / strongarmed. Online contacts will be prodded to comment as and when I blog about my vague forming ideas.
Come up with a grandiose title and some "poster" blurb. Design a poster of some sort.
A system that fits, with or without tweaking, and a setting made up or grabbed from somewhere.
Buy a new A5 notebook which goes in the pocket of my book bag for noting stuff I think of on the bus or hanging around the society rooms.
Doodle. Also prod around Google Images and Deviantart for stuff like the visual references I have in mind.
Arrive ready to accept players, and grit teeth in case such do not materialise. In the event that they do, run game. Adjust throughout based on player input or new dumb ideas that distract me.