There is not really a guide for it. I just destroyed every one of the environment objects and it dropped the little scroll for the art pieces. I kept on collecting them until i finished the DLC. On the Stats page it told me i had about 97% of the art collected. Once I finished, went and beat the final boss, the game took me back to the DLC to continue to get XP and collect anything i missed, just like the main game. I continued to destroy the environment object and it eventually popped. hope this helps.