Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 133)

3.
Select a custom white balance number to be registered.
Press < >.
Turn the < > dial to select a number from 1 to 5 for < * >, then
press < >. The custom white balance will be registered to the
selected number.
4.
Select [Select image on card].
Images recorded on the card are displayed.
5.
Select an image to register as the custom white balance.
Turn the < > dial to select the image captured in step 1, then press
< >.
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