I have been the stepmom of my husband’s child now for over 7 years.

I have been the stepmom of my husband’s child now for over 7 years. I live with the biological father. What do I have to do to become a legal guardian of this child. Father has legal and physical custody. The biological mother lost custody over 7 years ago due to drugs and only has supervised visits now and does not use them. She is homeless and has been for months. We cannot track down a physical address for her. How can I file for guardianship? Can a power of attorney work signed by the father?