In Indian English, sometimes we say things like this: 'Do one thing. Take a left turn there'. Basically, I am asking somebody to do something, which comes in following sentence. What's the better way of saying so?
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"Do one thing" has a hint of being patronizing. Don't use it when addressing someone who is older/superior to you.
This way you've got their attention and your English right :-) But like Barrie said, if it's commonly used by people, then there's no reason not to use it.
If that is what is found in Indian English, then there's no reason not to use it. In British English we might precede the sentence with something like 'Now' or 'Right' or 'Next thing'.