The first person created a character and give it a name and gender. He/She then says what happens in its first year of life.
Then the second person says what happens when he/she was two years old.
...
When the 100th person says what happens in the character's 100th year, he/she will say how the character died.
Then the 101th person will start all again. don't start a new character when the previous one hasn't died!
E.g
1: Girl, Kelly Black
1 year old: born in a wizard family, eldest daughter in the family
2: 2 years old: accidentally knocked over a vase, got a scar on her neck.
3: 3 years old: got lost while playing outside, have to live on her own.
4: 4 years old: beg for food, very poor
5: 5 years old: first magical incident, seen and nearly caught by a muggle
...
10: 10 years old: receive the invitation of Hogwarts, go to the Diagon Alley
...
100: 100 years old: died because of a strange, severe disease which the St.Mungo's Hospital failed to cure.

1 year old: Melody Jones, girl
born in a witch family, third child in the family.

2 years old: Accidently, got bullied by a muggle boy, caused indelible detest to muggles.

3 yrs old: tried to hurt many muggles which caused her parents to separate her from the outside world

4 years old: hurt another muggle, abandoned by her parents
Oh, poor girl

5 years old: go to an orphanage. Bullied by other kids.

6 year old: vowed to become the next greatest dark witch & hunt down muggles

7 years old: first instance of magic, set a desk on fire. wait, isn't this the next voldemort?

8 year old: bought a wand and started practicing spells

9 years old: got into a fist fight with a peer at school and suspended