For drag and drop, drag the file you want to register from Explorer and drop it onto the button where the submenu is created.

For menu-based registration, right-click the button with the submenu and select Register Item from the displayed Item Menu.

Items registered in a submenu are added to the bottom of the menu. Reorder them as needed by drag and drop.

You can move items from a submenu to a page, or move items from a page into a submenu.
Moving Items

You can select an item by typing its name on the keyboard.

Hold down the Ctrl key while clicking items to select multiple items, or hold down the Shift key and click two items to select a range, allowing you to perform subsequent operations on all selected items.

When multiple items are selected, double-click any selected item or press the Enter key to launch all selected items at once.

Left-dragging items in a submenu moves the items, while right-dragging performs OLE drag and drop. Item movement can be performed with multiple items selected, but OLE drag and drop can only be performed on a single selected item. These behaviors are specific to submenus and differ from the Folder Contents Menu.

Start dragging items without holding down any modifier keys. (The mouse option settings Drag-and-drop are not applied.)
When copying items, press the Ctrl key after item movement has started.

When you drag a file from Explorer and move the cursor over a submenu button, the menu opens automatically, allowing you to drop the file onto an item in the menu.

Various menu operations can also be performed using Shortcut Keys and Mouse Operations.

Settings related to submenus can be changed in the Menus options.