It's a simple technique, really. I meter for the sky in manual mode and make sure to set my shutter speed below my flash sync speed which is 1/250 sec. I slightly underexpose the sky. I then use my flash off-camera using its wireless remote feature. The camera's TTL handles the rest. That gives me a good base exposure and then I refine the look later in PS, but the core of the image happens in-camera.
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