Fair enough. I was once terrible with women.
You can be direct or indirect. If you receive obvious indicators of interest, direct is fine -- "Hey beautiful," "My eyes are up here," etc. Indirect would be more neutral: A comment on the environment, an observation, a question. Honestly it's not what you say, it's how you say it. You can say seemingly harsh or blunt things with superb effectiveness because your tone is playful and warm and you have a smile on your face.
What you say means little, maybe twenty percent. But being flirty -- not being afraid to touch, which is key -- confident, strong body language (leaning back a bit, back straight, broad shoulders, relaxed smile), playful and humorous. Passionate, honest and interested.
If the girl/woman finds you attractive, it will be fairly obvious. She'll laugh at things that aren't particularly funny, she'll touch her hair or otherwise preen/adjust herself, her pupils will dilate, bat her eyelashes, etc. In my experience if you're attractive, confident and respectful, you're good. Just don't be afraid to push boundaries -- two steps forward, one step back.
Truthfully I don't know what I'm going to say until I say it. It doesn't really matter. Just be a man!