Cold Approaching, as in:
- no prior meeting
- no introduction (usually from a mutual friend)
- no affiliation (work, hobby, etc.)
- no existing situation for meeting random people (mixer, potluck, speed dating (does this still exist?), etc.)
- they have said nothing to you, shared a furtive glance multiple times, or some other extremely obvious outward expression of attraction (this is very rare unless you are a movie star or incredibly good looking)
- starting a conversation with the explicit intention of picking them up
Has a near zero chance of leaving any kind of impression other than outing you as a creepy bastard. It's a heartbeat away from cat-calling.
Regardless of what you think, most [fill-in gender\sex\expression combination you are attracted to here] are just trying to get through their day. Ya know, a human being. Like you.
Having said all of that, I'm not suggesting that you never talk to a person of the [fill-in gender\sex\expression combination you are attracted to here] in any situation. By all means, if you know someone that you are attracted to in one of the reversed situations defined above, say "hey", "how ya been?", "crazy party right?", "man [authority figure] is such as asshole, right", etc. Strike up a conversation. Build a rapport, and ask them out if it's not an inappropriate situation (position of power, event where you really shouldn't be picking someone up, etc.).
It's not that hard, really.