You can't. People can only walk their own path themselves. All you can do is assist and support - which also involves not constantly coddling her fragile self-esteem, but also use occasional criticism.
I think she knows that you like how she looks but she isn't sure about everyone else. Maybe you could get your guy friends to help out. Like maybe get her to overhear you guys talking about how beautiful she is. She will hear that and then think that maybe more people think she is really great and will build her confidence.