How would you co write a book?

i told my cousin i was working on a book series, and he wanted to know if i was intrested in co writing a book with him. we both write in the fantasy genre but how would we go about doing it? is it even possible? and how can we make it work when we live in two seperate states? i'm intrested in helping him and i think this would be a great experince for me as a writer but i just want to know what i'm getting myself into before i jump in