How do saints perform miracles?

I read that to become a saint something along the lines of praying to them and then being cured of an incurable disease needs to happens. So my question is it OK to pray to someone who is not a saint yet or may even be living? So for instance if I prayed to Paul McCartney and my lifelong acne went away, and I got a doctor's confirmation, could I propose him for a fast-track him to saint-hood once he dies?