Turn a knob on the front panel or tap on the corresponding rectangle on the screen with the parameter name you want to control. In the context menu that appears afterwards, press MIDI Learn CC.
Move your external control or tap the field and enter the CC number. Now the device will change the value upon incoming CC messages.
To control volume, press Layer. Tap onto the Vol rectangle in the upper left corner. Assign your CC as described above.
You can also go to Global>MIDI>Mappings and assign your CCs there.
Note that the assignment is valid globally for all patches. Make sure that you're sending the CCs on the correct MIDI channel so that you change the values on the correct layer.