You don't want to tell her directly, because this would be weird and awkward.

Instead, you want to imply or subliminally suggest that she is beautiful. You want to get into her head and keep her guessing.

— “Hey, I think I like you.” — “What? Wow! That's really creepy!”

Instead, you want to talk about one of her interests or something that you both have in common and then randomly state, in a low tone that is almost inaudible,

— “By the way, I like you.”

After you state this, change the subject or continue talking about what she is interested in or what you both have in common.

She might try to interrupt you

— “What did you just say?”

The correct response is to deny everything and to continue talking

— “What? Nothing. Anyway, …”

Shortly thereafter, you should make up an excuse about why you have to leave and let her brain process what just happened.

— “Look, I really have to go now. Nice talking to you.”

From here, you just need to smile at her the next time you see her. You should be able to figure it out.

Good luck!