If you are from a big city or frequent the malls a lot, how the hell haven't you done this? For me it was part of becoming a man. We used to challenge each other to see who got the most numbers. After your first or second rejection, it feels like nothing. The more you do it, the more you learn to read body language and can tell who will go along with you, and who won't. All you have to do is converse about simple shyt like where are they from, tell them where you're from, speak about where you are both at, at that moment, then just say "let's exhancge numbers so we can get to know each other better". Its that easy.
a lotta times when they not down they'll hit you with "I got a boyfriend" or "aw you're sweet but I don't give my number to strangers". So even when you get that L it doesn't feel like an L. But a lotta times they'll just give you the number. Girls don't really get approached like that in the street so there's a good chance you'll get it. I advise all my brehs to approach an attractive female they see. It's a numbers game.