here. The idea is fairly simple: one cartoonist is X the other is O. You lay out a 9-panel tic tac toe grid on a page and play tic tac toe, only instead of simply putting an X you need to draw a comics panel that incorporates the X in some creative way. The game proceeds alternating players until the comic is finished. (Keeping track of who wins is optional.)
|Tom Xs and Os...|
|...while Matt Os and Xs|
|game record of the above comic|
PS As Tom points out in the comments, what we did to save time (and ensure a nicer looking final comic) was to pencil only during the jam session (as in the pix above), then we each took one home to finish up and ink.