When opening the Sandbox Editor for the first time, not all preset toolbars are shown. By right-clicking the toolbar area, a context menu is opened in which you can show and hide several preset toolbars:
Pic1: Toolbars Context Menu
The Customize option in the context menu opens the toolbar customization window which allows you to customize existing toolbars and create your own toolbars from scratch:
Pic2: Customizing Toolbars Window
In the top you'll find a search bar:
Pic3: Search Bar
This bar can be used to filter actions in the window below. For example, typing "display" will filter out the "Cycle Displayinfo" action, which lets you toggle between displaying different amounts of debug information in the top-right corner of the Viewport.
Under the list of actions is the toolbar editing section:
Pic4: Toolbar Editing Section
In this section, you can switch between the existing toolbars, add new toolbars and remove them. You also see a preview of the toolbar with all the buttons that are on the toolbar you have chosen.
Right-clicking on the toolbar preview area will display a context menu:
Pic5: Toolbar Preview Context Menu
With this menu, you can perform the following actions:
The bottom section of the window is where you can edit the buttons:
Pic6: Button Editing Section
Depending on what kind of button you have selected in the toolbar preview, different options appear in the bottom of this window: