Create a folder on the internal sdcard called Media, inside that make a folder called Audio, then make another folderr in that called Ringtones (capital R). And place your ringtones in there. Same can be done with Notifications. You can use a file browser from the the play store to create the folders, or as PC. You will need to reboot for the mp3's to become visible.
You can also use an app like zedge to download and manage your tones.