Open the GarageBand app on your iPhone
Create a new project and choose an instrument
Tap the Tracks view
Import or record the audio you want to use
Trim the audio to 30 seconds or less
Tap the arrow icon and select My Songs
Press and hold the project, then tap Share
Choose Ringtone
Name the ringtone
Tap Export
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new ringtone