Women love compliments. No matter whether you're telling her she's good at what she does or that she looks amazing, women like to get a little positive reinforcement every now and then. If you know a girl who you want to make smile and boost her confidence regardless of how she feels, tell her that you think she's beautiful inside and out. Your encouragement is likely to keep a smile on her face and warm her heart.
Notice something about her that you can compliment her on. Look at the way she is dressed, the manner in which she's doing an activity or things she says as the subject of your compliment. If you plan on having a long talk with her, you can focus on more than one aspect of her that's beautiful.
Get her attention. Be sure you have direct eye contact with her so that she knows you're speaking to and about her and that you're serious. Strike up a conversation with her if you're shy or feel as if you need a way to work toward the topic of telling her.
Tell her slowly. If your purpose is romantic, use a low tone and use slight body contact by placing a hand on her arm, hand or shoulder. Say it sweetly and direct such as, "You are so beautiful. Has anyone ever told you that?" If your purpose is friendly, use a more casual approach such as saying, "You know, you're such a great person, you have a kind heart and your smile is amazing. I think you're such a beautiful person."
Use only a few supporting statements to further emphasize your point such as "You really are..., I like it when you ..." Be confident and sincere in your message, deliver it and leave it alone. Avoid going on and on to reinforce your point so that your compliment comes off as genuine and not overdone.