Transformation from the List monad to the Bag monad on the 2-Category of Groupoids

Question

Kock has shown that the Bag monad and the List monad are polynomial on the 2-Category of Groupoids. He even suggests there is a transformation between them (I think, in section 3.10 Examples) going from List to Bag. Can someone present the transformation from List to Bag in gross detail? It seems like I want his short paragraph expanded and given much more detail. It has been pointed out that this transformation is easy to understand, we simply forget the ordering of the lists. Can someone explicitly state how you do this with transformations between monads? It would also be nice to see this reflected in the functor between the category of free commutative monoids and the category of free monoids.