Don't start talking to them with the intent to ask them out. Just see if you can get their attention, see of you can find a little bit of common ground to build a conversation on, then, if they are receiving you well over the course of your conversation, say something like "hey, I enjoyed talking to you, would you mind meeting up some other time?" or something to that effect. This works best in places for socializing, bars, music venues, hobby spaces, etc. It can be applied to more formal places, just has to happen over a broader period of time, as you get to know your coworkers, or whatever.

The strong vibes of "my intent in talk to you is to have sex" is, often, what makes things uncomfortable. So, even if you can't find common ground, or the conversation went well and she turns you down, or gives some non-committal answer, and you part ways amicably, you will have not be seen as some creep.