Open the Music app or Files app and find the song
Trim the song to 30 seconds or less using GarageBand, iTunes, or Finder
Export or save the trimmed audio as a ringtone file
Open GarageBand on your iPhone
Create a new project and import the song
Edit the track to the desired ringtone length
Tap the Share button
Select Ringtone
Name the ringtone and tap Export
Choose Use sound as…
Select Standard Ringtone or assign it to a contact
Or go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the new ringtone