You just add three words at a time to the story that already exists. But since this is going to be the game that does not require that you were here from the beginning of the game, you MUST MUST MUST copy and paste what's already been said from the person above you's post. It's that simple.
Here's an example (not from the guild, by the way!!):
Me
So it is
and then:
You#1
So it is a long time
...
Someone Else
So it is a long time since turnips could
Hopefully you get it.
I'll get it started.