The final configuration of the toolbar is visible on the right pane while the available buttons that have not been used yet in the toolbar are visible in the left pane. To add buttons, highlight a command in the left pane and click on the right arrows >> while to remove buttons from the toolbar, highlight the desired button in the right pane and click on the left arrows <<.
To change the relative position of a button in the toolbar regarding the other buttons, highlight it and use the Move Up and Move Down buttons.