Apocalypse how: Your guide to handicapping the end of the world
Method of destruction: Aliens!
How that's going to work: They're going to come down from space, announce our imminent doom and then start blowing the living shit out of everything and everyone in sight. Laser beams rain from the skies. We're all rounded up and shipped to the pens beneath DIA. Not even Will Smith can save us, because again, that guy is an actor, too. Also, the kind of technology that would allow aliens to travel light years to reach us would be so far beyond our own that our technology would like toys in comparison. Maybe if we're lucky they'll prefer to keep us as pets or something, rather than just engineer a virus to kill us all while we sleep.
What it's going to look like:
Why that probably won't happen: It would mean all those UFO nuts were right about something, and c'mon, how likely does that seem?
Odds: 20 to 1