Here are the steps you need to take to use DEVL Route:

• Download the plugin from here.
• Install the plugin using the Joomla! Installation manager.
• Activate the system plugin Devl Route.
• Go to into the menu you want and edit the menu-item you wish to add a custom URI to.
  Note: The menu-item should already exist, Devl Route will not show up upon adding a menu-item, only when editing a menu-item.
• In the right column of the screen, you will see that an extra panel called "Devl Route" has been added.
  Open up the panel.
  
• Enter the custom URI you wish to use for this menu-item.
  The custom URI should not include the host of your site, it will automatically prefix the root of your joomla-website.
  E.G.:
  Your website is "http://www.my-joomla.org"
  and you wish to have "http://www.my-joomla.org/my-page" for a certain menu-item
  then you fill in "my-page"
• Save the menu-item and you're done.