On web, you can add a widget by clicking on +Add button at the top right corner on Today’s screen. In mobile devices, click on + button to add the widget.
To hide the widget, click the cross icon visible at the top right corner of the widget. To rearrange the order of widgets on web, just drag and drop. On mobile, select the edit button at the top, and press and hold to rearrange the order of widgets.
Also published on Medium.