G5X II back button AF lock

How do I set back button focus on the G5X II?

I can get to wrench, 5, Custom Functions, C FN II:Others Customs Controls, but I'm not clear what to do next.

You pick the control from the list that you want to use for the function, press SET/OK button to confirm the selected control, and then you should see icons of available functions that can be assigned to that control.
I must say though that I didn't find that function on that particular button on my M6 II either. Maybe not all buttons are allowed to do that.

I realized I have to pick from the list. The questions are which to pick and how to operate after picking.

Well, the one (button) that feels handy and ready, and the usage is that once you are focused, you don't need to wory about losing focus for the static point for example, if you recompose a little. You know you stayed focused at that place. Also you know that changing focus would not move with exposure that much at all. And so you can stay busy with the settings you are currently managing, and nit being worried about everything at all. With the main shutter button, if you are not snapping, you rather always need to check composition, exposure and focus, which is often not that wanted shooting process.