There are no rules of friendship, at least not in a book.
No law that has been written, do not bother to look.
But this I know about friendship, it doesn't need a rule.
Just be someone you'd like around yourself and everything will be cool.
Be the best of company, but also be willing to be guided.
Give love and dare to take some too, friendship is never one-sided.
So if you really want a rule: be the best friend you can be.
Then you will surely succeed to find a friend who will fit you to a tee.