You'll first want to create a navigation - this contains your navigation nodes with links to elements or custom URLs. Head to "Navigation" on the main sidebar menu, and click "New Navigation" on the top-right.
Once saved, click on the navigation title to start adding your nodes.
Using the right-hand side menu, add either elements or custom URLs. Once added, they'll appear on the left-hand side in a structure. Hover over any navigation node to see some actions - move, edit and delete. Any changes you make to this structure will reflect the changes on your live site.
When linking to an existing element, the Title and Enabled settings will always reflect the original element's state. So, if you change the linked element's title or disable it, it'll reflect this change in the node. You can override the Title and Enabled settings to become independent of your element changes - if you wanted a custom title for instance.
Available for Craft 3. Get it from the plugin store.