How can mankind be the stewards of God's creation?

Anyone can be stewards (or managers) of creation. You just need to look after God's creation properly and then you are His steward. Do things like looking after the environment, caring for animals, maybe possibly SAVING animals extend a kind hand to a fellow human being, etc..

You just need to provide common sense and be thrifty when necessary. But remember, this world is for mankind and not mankind for this world. Natural Resources can and should be used and managed properly.