Flirting advice please

So today I was sitting outside a restaurant and two attractive women seem to be looking at me as they walked by. Im a bit socially inept and especially in these situations. I found them attractive so wanted to signal this but I have two questions on this.

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1. Is there a reason women look with a kind of blank expression. They don't really smile. I know people are different but this seems to happen every time.

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2. I'm quite low on confidence and struggle with eye contact but am I supposed to lock in with their eye contact and smile or make a neutral expression lol. The first girl who walked past seemed to be fully staring and I looked at her quickly then sort of looked to side and smiled. That's not the play right?

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