I don't stare when a nice-looking woman walks into the room. I try to act like I have seen it all before. I glance that way, then look elsewhere. Then occasionally look over that way again once in a while. If I'm not planning to hit on her that's it. Otherwise, I try to meet eyes with her and give her a smile, then go over and talk to her in a bit.
Yes, a woman needs to be at least slightly direct to let a guy know she is interested. Men tend to think about things in clear, direct terms. Women try to make things too subtle and complicated and guys just don't catch on easily. I read about a study once involving MRI scans. They would ask people questions and see which parts of their minds would light up with activity on the scans. Just before a guy would answer a question, there tended to be just one section of the brain that would get active. Probably the section of his brain where relevant information was stored. Women's brains would light up all over, like they were connecting all kinds of information to that specific question, both relevant and irrelevant info. Anyway, a woman can encourage a guy by asking him questions about himself and dropping a hint or two that she isn't currently in a relationship. One way would be to make some comment about his girlfriend and see how he reacts. If he is a decent guy, he will mention that he's married or dating someone. If he isn't decent, you might catch a telling moment of hesitation before he lies about his relationship status.
I flirt with a women with a mixture of attention and flattery. Not too much, just enough to show some interest. If she isn't interested, a woman will tend to drop a heavy hint, like suddenly mentioning a boyfriend or husband while answering a question.
Sometimes I randomly flirt with my girlfriend if the moment seems right, and sometimes I do it when I'm trying to get her in the mood for sex.
Good decisions come from experience. Experience comes from bad decisions.