First of all, copy your custom song file by downloading it from your mobile, or transferring it from your computer.
Next, use a file manager to navigate through your mobile's storage and search for your transferred song. If you do not have a file manager currently installed, you can download this application.
Once you've found your custom song file, select it and choose to move it to the Notifications folder located in the internal storage of your device.
Then, go to your phone's Song & Notification settings and select your custom song in this list.
Hope this helps.