How to add a new button to the button bar in Total Commander for Android? I got Total Commander on my Android phone.

I need to check the directory size for a folder and the solution is:

Add a new button to the button bar, and then..

I understand that but my question is how to add a new button to the button bar?
Tap on icon (arrow). You get a +, add details as per link - you can see the button with KB, on the first line