think about it, we already know she is smart enough to pull it off (look at how she overthrew the earth king and outsmarted long feng), she's got the most powerful earthbenders on her side, she knows about the day of black sun, knows her way around the fire nation, and has an inside track to the fire lord.the odds are on her side.
heres how it could happen... weeks before the day of black sun she shows up with a small group of dai lee agents with zuko and iroh in hand, naturally??ozai welcomes her back. she tells??him about??how she took over ba sing se and??the eclipse,??she then??offers the agents as body guards in case anyone attempts to invade during the eclipse. little does he know she just made him a prisoner in his own palace. and as the??eclipse approaches she has the rest of the dai lee in position to invade. now ozai has nowhere to go and his only choices are to surrender the throne or death. this would make her ruler of the fire nation and the earth kingdom. aang better watch his back.
??let me know what you think, and feel free to add on