Read the situation accurately. Sometimes, friendly flirting can be an effective way to have fun and show warmness toward someone, although occasionally it may be considered inappropriate. Before flirting, think about whether the person would appreciate flirtatious behaviour, or whether she might interpret it as being offensive, and thus unwelcome.
Compliment the other person in a playful way. This compliment should ideally be about something superficial, otherwise you may appear too serious and romantically interested. Unless you're confident that it wouldn't be inappropriate, focus your attention on something which isn't related to the person's body -- for example, compliment her fashion sense or perfume. You should ideally make this comment in a location in which other people are present and within earshot, as this demonstrates that you're not making a serious flirtation in private.
Wink while making a "clicking" sound or make a similar gesture to suggest that you're just being playful. Doing so can further help eliminate any miscommunication, and prevent the person thinking you're genuinely saying anything with a romantic intention behind it.