After you have created your own Custom Facebook Tab, you can add this tab to your Facebook Page* with the use of the Facebook Tab Installer application. After installing the application from Left Menu > Add-Ons, you can find it under Left Menu > My Add-Ons.
In order to add your tab to Facebook, access the application by hovering it's icon under My Add-Ons and clicking on the cogwheel icon, select your Facebook page from the drop-down menu (you must be logged-in to Facebook), enter a name for your tab and click the Install Facebook Tab button.
Please note that you can add up to 4 tabs per Facebook page. After the 4th tab, the application will allow you to overwrite an existing tab.
Once the tab is added to Facebook, it will be displayed on your Facebook page:
*If you don't have a Facebook Page set up yet, you can learn how to create one here, or go ahead and create one here.