Open the GarageBand app on your iPhone
Import or create the audio file you want to use as a ringtone
Trim the audio to 30 seconds or less
Tap the Share button
Select Ringtone
Name the ringtone
Tap Export
Choose Use sound as…
Select Standard Ringtone or assign it to a contact
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Select your custom ringtone
