How to convince an insecure girl she's beautiful?

You got to prove it to her. Actions speak louder than words. Sometimes the reasons people are insecure is because other people make them feel that way by telling them whats wrong with them. Show her that what other people think or say shouldn't matter. She should first accept herself before anybody else can and should know shes beautiful just the way she is. Nobody is perfect and nobody should feel ugly. There's always something beautiful in somebody.