First of all, we can find a topic from the form of girls. For example, "You have lost weight recently." This is what many girls like to hear. Even if everyone knows what you said is a lie, girls still enjoy it. Everyone likes to listen to good words, which is better than saying, "You have gained weight again." Much better. Then from this, it extends to slimming, physical exercise, slimming products, consumption power and so on.
Secondly, then, you can say "your clothes are really beautiful", if the clothes are really bad, say "this dress is really special", "whatever clothes you wear looks good" and so on. Extend to dressing and shopping. Understand their understanding of boys and learn how to match clothes for boys.