Canon EOS-1D X Mark III Advanced User's Manual page 131

4.
Shoot a plain white object.
Look through the viewfinder and aim the outlined area over a plain
white object, so that white fills the area.
Focus manually and shoot with the standard exposure set for the white
object.
You can use any of the white balance settings.
The custom white balance is registered to the camera.
To shoot with the registered custom white balance, see
Shooting with Custom White
Caution
If the exposure obtained in step 4 differs greatly from the standard exposure, a
correct white balance may not be obtained.
Balances.
131
Selecting and