A flyout consists of nested commands under a button. The flyout button is recognized from the inverted triangle located at the right bottom of a button.

The procedure to create flyout toolbar is as follows:

1. Go to Tools > Customize Interface.
2. Expand Toolbars.
3. Right mouse click to select New Toolbar.

4. Rename it.
5. Drag required commands to the new toolbar.
6. Select the toolbar and Right-click to select New Flyout.

7. Rename it.
8. Drag another set of commands under this flyout.
9. The sample customized flyout is as shown in the screenshot below:

As you notice, it is  available as a separate toolbar, too.

In order to activate it in UI, RMB click on toolbars > Toolbars.
Check for the newly created toolbar.

In the UI, in order to see the commands in flyout toolbar, press and hold down the button with inverted triangle.