First, the cards themselves
I choose these because they are not paper, not a blank sheet. The writer is therefore less likely to feel that the first sentence she writes is the final draft of the first sentence of the completed piece. No "blue-book syndrome." Second, the order. I have alternated the individual and the social: writers work alone, and they work with others. Ideally, a writer chooses when to work alone, when to get responses from others. Practically, in a class of 24 writers, one stages it.
Another note on the order: I alternate between the centrifugal (center-fleeing) and the centripetal (center-seeking). Peter Elbow has called this "loop writing." I'm assuming here that at times as a writer you need to write away from your center. Peter has called this "the voyage out" and that at times as a writer you need to re-focus, to come back to your center ("the voyage back"). A note on your second class: if history predicts, you will have several new students in your second class. This means that to a degree you will be conducting two classes: one for those who were there for class I, and another for the students new to your section.
A slick way of dealing with this: have the veterans read and respond to one another's mid- process drafts; and have the new people do the card exercise. Before the end of class, set up pairs of new students and have them schedule a time to meet and read/respond to one another's writing outside of class, before the next class. Then, voila! By the beginning of class 3, everyone is on the same page.