Open the song in the Music app or Files app
Trim the song to 30 seconds or less
Save the trimmed version as an AAC file
Rename the file extension from .m4a to .m4r
Connect the iPhone to a Mac or PC
Open Finder on Mac or iTunes on Windows
Drag the .m4r file into the iPhone’s Tones section
Sync the iPhone
Open Settings on iPhone
Tap Sounds & Haptics
Tap Ringtone
Select the new song ringtone