Open the GarageBand app on your iPhone
Tap the plus button to create a new project
Choose Audio Recorder, then tap the Tracks view icon
Tap the Loop Browser icon
Import or select the song you want to use
Drag the audio into the timeline
Trim the clip to 30 seconds or less
Tap the down arrow in the top-left corner
Tap My Songs to save the project
Press and hold the project, then tap Share
Tap Ringtone
Enter a name for the ringtone
Tap Export
Tap Use sound as…
Tap Standard Ringtone, Text Tone, or assign it to a contact