In Visual Studio 2019, right-click the toolbar area and choose Customize. Then on the Commands tab, select the Toolbar radio and choose one of the toolbars you have visible (say Build). Here you can add or change the order of commands.
In older versions of Visual Studio (eg. Visual Studio 2003), while in Customize mode you could grab and drag a button to insert a separator.
How do you add toolbar separators in Visual Studio 2019? In Customize mode, you cannot grab-and-drag buttons and I wasn't able to find any Separator option when I click Add Command....
