tell her. her eyes sparkle like the nighttime sky, her skin is as soft as a cool spring breeze, her smile lights up the world, and her smile can calm an angry beast. try some of that on for size, hope it works. i don't know what kind of music that you like , but give a listen to Stardust by Willie Nelson, one heck of a love song
You can come right out an tell her. You can tell her she has gorgeous eyes, shiny hair, a wonderful personality, she puts a smile on your face and those are just a couple of things that make her the beautiful person she is.
ask her to stand in front of a mirror and ask her what she sees and when shes done, stand behind her hug her and tell her what you see in greatest detail about how much she means to you and how beautiful she is from the head on her hair to the paint on her toe nails