Sure they work, in just the same way as a white card does, or a grey card - basically any neutral subject that you take a reference picture of under the prevailing light conditions, then tell the camera to set itself so that that neutral subject is reproduced as neutral. Job done. That's what custom white balancing does. I've used a white door and a white tablecloth before
Just fit it over the lens and point it at the light source. There's no magic to them.
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