Summarize:

This will make it easier to whisper in her ear, which would be awkward directly from the front—it could appear as if you're going in for a kiss. If you'd still like to add some element of surprise, wait until she turns her head away from you before approaching her ear.  If you already know her fairly well, you could try to surprise her by whispering from behind her. But do this only if you know she's not easily frightened, since it could produce a big scare. If you especially want to avoid startling her, you can try placing one hand gently on her shoulder before approaching her ear. If you're on her right side, use your right hand to cup your mouth. If you're on her left side, use your left hand to cup your mouth. This way your whispering will be shielded from the hearing of any others who might be in front of her or to her side. If her hair is covering her ear, avoid brushing it away if you've just met her that day. If you've been on at least a couple of dates, brushing her hair aside should be acceptable. If you've only just met the person, avoid making direct contact between your cupped hand and her ear. If she's your girlfriend or if you've been dating for some time, it should be okay to press your hand lightly against her ear while whispering. If you're in a noisy area such as a busy bar or at a party, you may have to speak above a whispering tone in order for her to hear you. Be careful not to yell, though. A tone slightly above a whisper should be audible, since your mouth will be close to her ear. But if there is little or no background noise in your area, speak slowly in a whisper. If she immediately moves away from you even before you get a chance to whisper, or after, you'll know to avoid trying to make physical contact with her for the moment. But if she responds with a smile, giggle, and sustained eye contact, you've done well. Yet be careful not to take a positive reaction as an invitation to move a step beyond by hugging her or holding her hand right away. Be content with having made her laugh and smile, and wait for a response from her, such as touching your hand or moving closer to you, that shows she's probably comfortable with more involved flirting.
Stand or sit to the side of her. Cup your hand around your mouth. Approach her ear. Ensure that she can hear you. Follow up according to her reaction.