If you just want a series of submenu items, then you do not need to use Mega Menus, just use a regular menu and submenu system
Mega menu are for people who want formatted content in their menu instead of the regular list of menu / sub menu items.
If you need help with using WordPress Custom Menu (in Appearance > Menus), Search Youtube for WordPress custom menus tutorials.
In Appearance > Menus, you can create your Menu(s) with any number of menu items and sub menu items.
Simply select the various elements / pages you want in it on the left side, and in the right side, organize them in the order you like.
To make certain menu items sub menus of others, just drag them slightly to the right so their position is indented