I’m going to give you an honest, ethical and principled (pun intended) answer, not some of the useless and dangerous pick-up artistry (little better than street harassment with specious evo-psych justifications) that you may get elsewhere.

1. Follow the Platinum Rule. (“Treat others the way they want to be treated.”)

That’s pretty much it, with some caveats.

1. If you are a male or male-appearing person, make extra, super-duper sure you read social cues to determine if the person you want to flirt with indicates they do not wish to flirt with you.
2. Accept any rejection with as much good grace as possible and depart without hurling insults, put-downs, or requests for why you were rejected.

This advice is good for everyone. However, since girls, women and female-appearing/acting people are harassed and sexually assaulted at a far greater rate than boys, men and male-appearing/acting people, it’s much more incumbent upon you to show you’re not going to get creepy or weird (read: potentially dangerous) when rebuffed, since you’re in the group that’s much more likely to do so.

If you think these precautions are overkill, then please blame (1) fucked-up male socialization, (2) rape culture proponents and apologists, and (3) the sexual harassers and rapists that make life miserable for everyone, but most especially for female-appearing/acting people, girls and women.

Yes, I realize my advice most easily applies to male-female/man-woman interactions. But the general rule applies to anyone (QUILTBAG or not) wishing to flirt.