There is no trick, you just do it. I can go down all the arguments>fuck what everyone else thinks, that will only hold you back>good guys will give you cudos for trying, even if you fail>you will gain experience and will be better when the next girl comes around
yada yada, you just do it. You are not a creep for showing interest in a girl. That's the most normal thing. And perhaps one day a girl might interpret your advances as creepy, just accept that. This land mine type of mentality that turns you into an in-active pussy is exactly what differentiates men from boys. Just accept that there are going to be girls who will interpret your advances as creepy, despite the fact you weren't actually being creepy. If she feels that way about you, accept it and move on, but don't let it stifle you. That's not even worth discussing since this girl gave you plenty of signals she felt comfortable with you.
I will give you a piece of advice. Make flirting with others something normal to you. Flirting is just playful jabs and you can do it with anyone. If that becomes second nature for you, it won't feel off or artificial once you are doing it, because you are actually interested in the person.