We can affirm her from three aspects, highlight her real advantages, help her discover beauty and enhance her self-confidence.
First of all, you can praise a certain part of her appearance. For example, I think your eyes are beautiful, as bright and full of aura as the stars in the sky. Or: I think your legs are beautiful, thin, long and straight.
Second, you can praise her overall temperament. For example, I think you have a slight bookishness, which should be "complacent with poetry as a book"
Third, you can praise her character. For example, I think you are like a sunflower, with bright sunshine, and people around you are unconsciously attracted to you.
To sum up, let her feel that you appreciate her from the heart.