From the Game's Introduction:
Find three to five players. You will each be going on a long journey, travelling far away from the others. You will still be able to stay in contact with each other, although as you travel, the distances between you will become greater, and the time it takes the messages to travel between you will increase. But still, you will continue to write, to maintain the threads of connection.
Each round, players write short letters to other players, alternating between writing a letter and passing letters on. The first letter from one individual to another arrives immediately. The second letter between that same pair will take an additional round to arrive, and so on.
After a dozen or more rounds (around fifteen to twenty minutes) stop writing letters. The remaining undelivered letters still in circulation get passed on to their targets, but these letters should be taken away from the game and read later on in private.