Its pretty simple, one person makes one panel and the next person continues it with a panel of there own.
Example:

Each person must only have one panel with one character in each post and they must continue or add on to the previous plot in some way.
Using characters from your comic is not compulsory.
You can contribute more than once alsong as you give time for other people and avoid overdoing it, However this is aloud if characters are attempting a one on one conversation.
High quality art is not expected because it is quite a hard game, and remember everyone has the right to contribute no matter how good they are.