Open the GarageBand app on your iPhone
Tap the plus button to create a new project
Choose any instrument
Tap the Tracks view icon
Tap the Loop Browser icon
Import an audio file, song, or recording
Trim the audio to 30 seconds or less
Tap the down arrow or My Songs to save the project
Press and hold the project, then tap Share
Tap Ringtone
Name the ringtone
Tap Export
Tap Use sound as…
Choose Standard Ringtone, Standard Text Tone, or Assign to Contact