Default answer: It depends on the situation. For my lady friends maybe joke around with them a little bit. When the time is right maybe make a sassy comment, like when my friend was helping me carry my coffee table up to my apartmet, and I was standing in front, I asked her if she liked the view. One in particular I am just terrible at flirting with.
Lady friends online? Much more to the point and openly displaying affection, but in a joking manner. Just casually saying things like "(Name), I wuv uuuuu." or "(Name) is so awesomeauce!" They respond surprisenly well. The other day on jumped in my party and said "Horono, I love you!" and left.
Lady friends I'm not very attracted to are fun. I'm much more confident around them and usually a lot more forward. Some I can get away with saying anything. One in particular who I eventually developed strong feelings for I could say ANYTHING to. It was only once I had feelings for her that I stopped being as cocky and things got weird between us. I eventually asked her out, and lafter a few days she gave me the "Don't want to ruin friendship" bullshit. At the time I was hoping she would say no, but looking back I am sadface about it.
Regular girls I run into, I'm not very good at flirting with. It's foreign to me how I can go from "Hello person I just met" to "Let's totally hang out sometime." O well I guess.