Save the song to your phone
Open your phone’s Settings
Tap Sound or Sounds and Haptics
Tap Ringtone
Tap Add Ringtone or choose a custom ringtone option
Select the song file
Confirm and save
Set it as your default ringtone
On iPhone, use GarageBand or iTunes to create a ringtone file if needed
On Android, move the song to the Ringtones folder if needed
Open Contacts if you want the song for a specific person
Edit the contact
Tap Ringtone
Select the song
Save the contact changes