What do I need to do to get guardianship over my 12 year old cousin??

My aunt lives in Wisconsin and I moved to Colorado. She is having huge issues with my cousin and I want to get guardianship over my cousin. She is not being physically abused or neglected. It's just we both feel that it would be better if my cousin came to live with me. At this point, I am unsure if it's temporary (one year or so) or until she turns 18. What would my aunt need to do and what would I need to do to gain guardianship of my cousin??