It works like this. You can read the story. At suitable points, there's a choice that you have to make, and the story continues according to the choice you made. Eventually, you get to the point where you make a choice, and up comes "Your Turn. You have reached the end of the story action. Now it's your turn to enter what happens next in the story." and at that point, it's over to you. You have to write the next part of the story, up to another decision point. There's two options, and you have to say what they are, such as "Sherry opens the box" and "Sherry decides not to open the box". Then someone else will write what comes next. Or you can write the next part too, if you want.
So, here's the story so far ...