The idea is to take a random object that is utterly familiar but considered unimportant and use it as an object of learning, finding the connections between that object and ourselves. (Reminds me of Marx's question in Volume One of Capital about how two objects can be made commensurable.)
Step one is just to take the bag and answer the question: What do you see? This step requires time and care, it should be detailed and very specific, a close description, a close reading.
Step two is to ask people what do you think of the object? Every answer must be verified by showing it in the object. E.g., "The object is useful" breaks down to size, shape, handles, durability, weight, cost...
Step three might be to compare the object to another object with which it seems to have nothing in common, e.g., a book, a song, a person, an economic system, and see how it is the same and different. The guiding conceit here is that every object is the product of labor, creativity, thought, intelligence -- everything is in everything, and everything is in a plastic bag.
Or, step three could be to do the "six degrees of connection" activity, trying to find what links the plastic bag to any other object of human activity.
Or you could do a "story of stuff" approach and imagine (act out) the production, distribution, consumption, and disposal cycle of the bag.