Canon EOS R5 C VR Creator Kit with RF 5.2mm f/2.8 Dual Fisheye Lens - Manual (Page 185)

4.
Shoot a white object.
Aim the camera at a plain white object, so that white fills the screen.
Set the camera to manual focus ( ) and shoot so that the white object
has standard exposure.
The custom white balance is registered to the camera.
Caution
Correct white balance may not be obtained if the exposure of your image differs
greatly from standard exposure.
Note
Instead of shooting a white object, you can also shoot a gray card or standard 18%
gray reflector (commercially available).
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