How it just flows. The idea behind leapfrogging small talk is to talk like you're friends already.
It's a bit of a Jedi mind trick but the basic idea is to prime the pump. When you walk up to someone point blank (especially a stranger, especially a pretty girl) - 'What are you up to?' - you're putting all the onus on them to carry the conversation, believe it or not. Because you didn't have the courage to actually start a conversation. They don't know you - so of course they'll say 'nothing' or 'errands. bye!' to politely blow you off. You didn't invest much so they won't invest much back. And the conversation will die.
Commitment is everything! Asking someone a question is not starting a conversation. Tell them something about themselves.
How to start a conversation? Practice. Compliments are great. If you notice something interesting about someone, instead of just keeping it in your head. Just walk over confidently and say it. "I like how you are so engrossed in that book in the midst of the craziness here".
Never state a fact without a feeling. Facts are boring.
"It's a pretty nice day. Hasn't been too humid lately."
"Yeah. Hey so what are you passionate about?"