… link 
As Justin says, before giving lots of helpful suggestions: There are no rules 
@Richard_close2u gives a pretty in-depth account of a collaboration I was involved in, which involved stark separation of co-writing ‘duties’.
It never would have entered my head to think I could co-write a song if it hadn’t been for a Christmas competition many years ago, which had to be a collaboration.
If you ever get the opportunity, it’s a great learning experience and can be bags of fun 