Install a third-party file manager from the Google Play Store (there are many free options available), open it and search for the music tracks you want to use as ringtones.
Copy a track you want to use and then paste it into the Ringtones folder, and repeat the process for any other tracks you wish to use as ringtones. If you choose the Move option, the tracks will no longer be available in the Play Music app, so make sure to only use the Copy function. Now go to Settings>Sounds>‘Phone ringtone’ and you will see all of the selected tracks listed as potential ringtones for you to select from. Many common music file formats are compatible as ringtones.