Just look her in the eyes and say "you're beautiful" and smile. She'll be like "aww thanks", so just smile and continue the conversation... I think, that'll make her think about you at the end of the day.
Also, don't say it too much (friend zoned guys do lolol) and don't hang on to it like, just continue the conversation as you normally would. :)
Don't say it like it's a casual observation. If you want a girl to feel like it's heartfelt, you need to say it during a serious conversation, and you need to have something specific about her appearance that you love.
If you are not dating her yet you should not tell her anything like that. She will know you want her and she will feel she has you wrapped around her finger. She will probably lose interest and go on to another challenge. You will do better playing a little hard to get.