Basically you're telling a story one sentence at a time, but you alternate "the good thing is" and "the bad thing is" every other sentence.
Example:

Me: One day, you were walking through the park
Next person: The good thing is it was sunny.
Next person: The bad thing is that you tripped on a rock and fell.
NP: The good thing is you landed on a pillow.
NP: The bad thing is the pillow was full of bees.
NP: The good thing is you escaped the swarm of bees by jumpin in a pond.
NP: The bad thing is you can't swim.
and so on ...

So here goes:

One night, you were driving down the street.

The good thing was it was a nice, clear day.

The bad thing is that by trying to impress some of your friends you did some kind of jest with your car, trip on a rock and smashed a van. It hurts. :P

The good thing was that the van was full with feathers . . .

The bad thing is that you're allergic to feathers

The good thing was that you jumped trough the window

The bad thing is you jumped out while driving along the edge of a cliff, therefore plummeting down the steep and rocky mountain towards the jagged and pointy stones below.

The good thing is that you had a parachute.

The bad thing is that the parachute has been slashed by your jealous ex-lover...

The good thing is that now you have a new lover