Start with only one or two obvious cards and start playing "what-if".
If the situation calls for a new responsibility, we either add it into an existing object, or create a new object to address that responsibility.
If one of object is too complex, we create a new object to assume some of the responsibilities.
Beck and Cunningham suggest learners to pick up the card whose role they assuming while "executing" a scenario. This will aid making a strong identification with the objects while describing their collaboration.