How would I go about adopting a child that my friend is willing to have for me?

I am medically unable to have children but I have a friend that she and her husband are willing to have a child for me and relinquish all rights so that I could adopt the child and raise he/she as my own. My friend is not currently pregnant, but she does want to help me to fulfill my dream of being a parent. This couple does already have children and plan on having at least 1 more for themselves, plus the one (or multiples) they would like to have for me.