how to make fun of a girl without insulting her?

Oh my god. NEVER call a girl the b-word and expect her to laugh. Nevernevernevernever. Well, okay, not until you know her REALLY well, well enough for her to not take it the wrong way.

The tip to flirtatious teasing is primarily to make the insult slightly flattering. For example, I'd get a good grade on a really hard test, and a guy who liked me would teasingly whisper "Nerd," but I knew he liked smart girls and from him, it was always a compliment in disguise. A lot of it is also in your facial expression and body language, so be sure you're sending all the right signals! You could give a girl the most heartfelt compliment in the world, but it your demeanor doesn't follow suit, it's hard to believe you. Make sure you come off as being genuine.

And honestly, just be nice! Girls will appreciate it more than you realize.

Perhaps the dirty looks were some sort of hint... Girls don't generally warm up to those who make fun of them. Maybe try flirting in a more kind way? I can see making fun of your close friends in a playful way, but it doesn't really work for picking up girls. I can tell you that it would not work for me.

Well first off find a girl who wont take things so seriously. And don't start calling her names on the first date, get to know her pretty well before you start really picking on her with "*****" and stuff. Never pick on a girls appearance, that's a bad area to rip on. ESPECIALLY weight. Just remember that stuff, and just think, "is this too harsh?" Before you say it. That's all :)