So generally for shadowing I add a Hue/Saturation layer and move the Lightness slider to about -60. Then invert the mask. Then paint on gently to add the shadow where needed.
It absolutely doesn't matter. They can be good, or crappy. Heck, they don't even need to be from your pro camera. They could be from your phone if you wanted! All that matters is that they're printed at a pro lab.
Calibrate in the daytime. Just close the blinds while you're doing it.
And you'll need to only edit in the daytime, until you have brighter lights that match daylight.