Basically, you choose an election to play in. Almost all are US Presidential elections, however, you could do something international, or even historical (I had one about the Confederacy, for example). Anyway, you can either pick characters for players to pick from, or they can choose their own. You'll basically act as a Game Moderator, making the scenario, polling, etc. The players will act as their respective characters and post speeches, campaing events, etc.
Hopes this helps.