Basiclly, one person begins a sentence beginning with 'Fortunately', then the next person starts a sentence beginning with 'Unfortunately'
e.g.:
Fortunately, he found the house in time
Unfortunately, the house was blown up
Fortunately, his family had gone to stay with friends
Unfortunately, there houses had been destroyed too
Fortunately, they had all decided to visit the funfair
And so on. Enjoy! (continue from there).