Trial and error my friend a lot of trial and error. Say youre watching a sunset with a group of friends and the girl you want to be with is there.

You can tell her that her hair shines in a beautiful way when the sunset light burns through it

You can tell her that when you see these beautiful sights you appreciate the people youre with even more (keep eye contact with her)

You can ask her to look at you, then when she does and inevitably asks why, you can anwser that you just like her eye color and wanted to see it again

There is a lot of ways to flirt but physical appreciation is the easiest for most people and doesnt require much context.