Open the GarageBand app on your iPhone 14
Tap the plus icon to create a new project
Choose Audio Recorder
Tap the Tracks view icon
Tap the Loop Browser icon
Tap Files or Music to import your song
Drag the song into the track timeline
Trim the song to 30 seconds or less
Tap the downward arrow in the top-left corner
Tap My Songs
Press and hold the project file
Tap Share
Tap Ringtone
Name the ringtone
Tap Export
Tap Use sound as
Tap Standard Ringtone or assign it to a contact
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select the new ringtone