Any single piece of content can have a menu item created which can be added to either your primary or secondary menus and which links to the content. To create such a link

1. Locate the "Menu Settings" tab at the bottom of the page while editing/creating content.
2. Menu link title: This will be the link text that will appear in the menu
3. Parent Item: The menu that the link will be placed into
4. Weight: Optional. In the menu, the heavier items will sink and the lighter items will be positioned nearer the top.

To manage a custom link after you have created it go to the "Menu" tab under "Settings"