Make sure that you are logged in to your site and while you are editing or creating content on your website (the process is the same if you are editing an existing item or creating a new one), scroll to the bottom and click 'Menu settings' to expand it.
Type in the title you'd like your menu to display, a description (good for search engines!), and then use the drop down box called 'Parent item' to place your menu.
The drop down box displays all of your current menus. The menu that you select will be the new menu's parent and so the new menu will appear below whatever you choose.
If your links are:
And you select 'Menu 3" as the parent, your new menu will appear like this:
The weight determines placement in the particular list. So in the above example, Menu 2.1 has a lighter weight than Menu 2.2. If you wanted to make Menu 2.2 appear above Menu 2.1 you would just give it a lighter weight - like -5 (negative numbers are 'lighter' than positive ones).