Open the GarageBand app on your iPhone
Tap Create Song or open an existing project
Tap the Tracks view icon
Tap the Loop Browser icon
Import the audio file you want to use
Drag the audio into the timeline
Trim the clip to 30 seconds or less
Tap the down arrow and select My Songs
Press and hold the project, then tap Share
Tap Ringtone
Enter a ringtone name
Tap Export
Tap Use sound as
Choose Standard Ringtone, Standard Text Tone, or assign it to a contact