How to make friends is entirely subjective because each one of us have different ways of associating with people and establishing relationships with each other. For me, its as easy as adding me as a friend! (That is why I have it open) and having people PM me to start a conversation up because I love it when people talk to me!
As for others, it depends. Helping others/offering opinion, engaging in roleplays, commenting/just talking to people with similar interests, etc.