Flirting with a girl is a delicate dance between coming on too strong and not giving her enough clear signals to indicate your feelings. Overall, flirting should be something that comes naturally -- if you find yourself forcing behavior, your potential love interest may notice the tension. If you already know the girl likes you then the hard part is over, so loosen up and play it cool.
Smile while you talk. Smiling is one of the most important indicators that you are enjoying another person's company and it makes you seem happy and positive.
Look the girl in the eyes. Eye contact visually indicates your interest with whom you are speaking.
Make jokes and keep the conversation light. If you can make a girl laugh, you are in a good position to build her interest.
Compliment the girl on something small and specific, like her hairstyle or a piece of jewelry. Avoid vague, general compliments.
Ask questions to show that you are interested in her. Just talking about yourself will send a clear message that you are not concerned with her history or opinions.
Initiate physical contact by touching her arm or hand while you talk. Physical contact helps make her more comfortable with you. Be careful not to be too aggressive or forward with your contact.
Stay calm and stay positive. If the girl already likes you, chances are good a polite, playful conversation will be flirty enough without the need for manual additions. The most important rule of flirting is to not overdo it.